How did the people know that John the Baptist was a prophet?

Easy! Anyone that foretells of future events is considered a prophet. A true prophet is one that forecasts correctly. A Hebrew prophet from God is anyone that correctly forecasts about events that influence the Jews/Israelis or their nation. Therefore John the Baptist was considered a prophet.

1. John the Baptist wore 'prophet's garb', which was sackcloth. (This was one indication) (Other OT prophets wore sackcloth as well)

2. Jesus called John the greatest prophet in the Old Testament, so Jesus Himself called John one.

Jesus Testifies about John Luke 7: …27"This is the one about whom it is written, 'BEHOLD, I SEND MY MESSENGER AHEAD OF YOU, WHO WILL PREPARE YOUR WAY BEFORE YOU.' 28"I say to you, among those born of women there is no one greater than John; yet he who is least in the kingdom of God is greater than he."

3. Sackcloth was made with animal hair, and it was something that was often worn while fasting. Prophets, wanting to be close to the Lord, fasted.

John the Baptist was an Essene ,separated group of Jewish Levite's. Kumron scroll confirms it .During that period Essenes lived away from cities their main job is to be pure to God ,their task to copy the Bible OT .people highly respected them hence they listened to John .

The people knew John the Baptist was a prophet because it was prophesied that a messenger or prophet would precede the coming of Messiah. Scripture records that an angel tells Zecharia John's father about this.

And he shall go before him in the spirit and power of Elias, to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just; to make ready a people prepared for the Lord. Luke 1:17

To be in the spirit and power of Elijah (Elias) was to be a prophet as was Elijah.

And they asked him [Jesus], What then? Art thou Elias? And he saith, I am not. Art thou that prophet? And he answered, No. John 1:21

Jesus wasn't the forerunner; John was, as Elijah.

In turn this fulfillment in John is of a prophecy that related back to this.

Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the LORD: And he shall turn the heart of the fathers to the children, and the heart of the children to their fathers, lest I come and smite the earth with a curse. Mal 4:5-6

To fully answer the question one would also need to understand the 70 weeks of Daniel and the timeframe of the expected Messiah. In other words, they knew a prophet like Elijah would come first. Suffice to say that some understood that Messiah would come by 5 BCE. This meant that the forerunner (John in the power/spirit of Elijah) would also have to be on the scene. This then is shown as mentioned above in Luke.

A prophet "foretells" or prophecies the future. In John's case, his job as a prophet was to tell the people Messiah would and had come. John was 6 months older than Jesus.

The voice of him that crieth in the wilderness, Prepare ye the way of the LORD, make straight in the desert a highway for our God. Isa 40:3

That's a powerful verse.

In addition, another function of a prophet was to annoint the king. A priest would also do this. And the trumpet would sound.

And let Zadok the priest and Nathan the prophet anoint him there king over Israel: and blow ye with the trumpet, and say, God save king Solomon. 1 Ki 1:34

That was the foreshadow of Christ, son of David, being annointed at His baptism "by" John (prophet and priest) and with the voice of God heralding.

And the Holy Ghost descended in a bodily shape like a dove upon him [Jesus], and a voice came from heaven, which said, Thou art my beloved Son; in thee I am well pleased. Luk 3:22

So, the people knew John was a prophet. It was prophesied. It was required within a certain time. It was fulfilled.

There are little of the expected cues and clues to clinch the question asked. What sparseness there is has been given. I can only add that there was an unusual yet genuine and ferocious loyalty demonstrated toward John which would confirm Jesus having accorded him- not greatest prophet status- but that of "greatest of men ever born of a woman".

Jesus was able to gain an advantage- knowing the fierce devotion of the Jews to John as the challenged leveled by the scribes, elders and priests to declare the source of His authority- He made them first answer about JOHN'S authority. They did not answer because "they feared the Jews". A wrong answer from them there would create more trouble than what they wanted for the answer from Jesus.

Can we glean from this that the Jews considered John the Baptist a prophet? With that kind of loyalty- I would answer yes.

For all the Prophets and the Law prophesied until John (Matthew 11:13).

Commentary - “... ‘until John’ means up to and including John ... before the inauguration of the kingdom .... The Prophets and the law prophesied until then and, implicitly, prophesied of the new era ... the kingdom itself ....” (D. A. Carson, commentary on Matthew 11:13, The Expositor’s Bible Commentary (Vol. 8), Frank E. Gaebelein - General Editor, p. 268).

But if we say, 'From man,' we are afraid of the crowd, for they all hold that John was a prophet." (Matthew 21:26) Commentary - “John was the last of a long history of OT prophets that looked forward to the coming of Christ.” (ESV Study Bible, p. 1842)

I wouldn't overthink this one, LukeoX.

God obviously called John to be a prophet and a forerunner to His Son Jesus, the Christ (Luke 1:17). Consequently, in obedience to God's call John presented himself to the people as a witness, called by God to be a testimony to the Light. Put differently, John was a light to the people, and Jesus was the Light of the world (see John 1:4-9; cf. 8:12). John's little light paved the way for the big Light.

When people believed John's message, they proved the sincerity of their repentance by submitting to his baptism. They then became John's disciples (see Matthew 9:14; 11:2; Mark 2:18; Luke 5:33; 7:18; John 1:35; 3:25; and 4:1). The angel Gabriel told John's father that his yet-to-be-born son would come on the scene "in the spirit and power of Elijah" (Luke 1:17). In other words, John's preaching ministry would bear fruit, just as Elijah's did.

Now, what all was involved in being one of John's disciples is not exactly clear to me, nor does the Bible say too much about that. Unlike Jesus, however, I don't think John was called of God to form a denomination such as--oh, I don't know--Baptists perhaps!? As the Christ's forerunner, he sort of faded into the background shortly after Jesus came on the scene. After all, John admitted humbly,

"He must increase, but I must decrease" (John 3:30).

Not too terribly long after John made that statement, he was beheaded by King Herod. Interestingly, however, just before he died John told his disciples to ask Jesus,

"'Are you the Expected One, or do we look for someone else?'" (Luke 7:19).

Despite John's greatness in Jesus' eyes ("among those born of women," Jesus said, "there is no one greater than John"), even he struggled with doubts about the identity of his cousin(?), the son of his mom's relative, Mary, whom John's mom called "the mother of my Lord"! (Luke 1:43).