What are the signs of a true apostle according to Latter-day Saints?

The reason you don't get an answer for this is that it's a little strange question for LDS. There is no good definition of signs of a true apostle in scripture, and Paul only mentions very roughly "signs and wonders" and "mighty works". The accepted answer to your question on BH.SE is good.

In addition, we are taught that signs come after the faith and not to look for signs.

https://www.churchofjesuschrist.org/study/ensign/1994/12/i-have-a-question/i-have-a-question?lang=eng

And Elder Boyd K. Packer, of the Quorum of the Twelve, has said, “I have come to know that the witness does not come by seeking after signs. It comes through fasting and prayer, through activity and testing and obedience. It comes through sustaining the servants of the Lord and following them.” (Ensign, June 1971, p. 88.)

Therefore, before asking the Lord for a sign or manifestation of his power, the righteous must exercise caution, remembering that “blessed is he that believeth in the word of God, and is baptized without stubbornness of heart, yea, without being brought to know the word, or even compelled to know, before they will believe” (Alma 32:16).

The scriptures, however, also make it clear that signs can confirm a person’s faith in the Lord (see 3 Ne. 1:8, 22; 3 Ne. 11:14–17); indeed, the Lord has promised that signs will “follow them that believe” (Mark 16:17).

He has said that when these signs are given to those who believe, they are given “for [our] profit and for salvation” (D&C 84:65–73; see also Mark 16:17–18; Morm. 9:21, 25; D&C 35:8–9; D&C 46:7–9). The scriptures record examples of people who asked the Lord in righteousness for signs or miracles and whose desires were granted (see Judg. 6:11–24; 2 Kgs. 20:8–11; Mark 9:20–27; Luke 1:34–38; Hel. 11:1–5; JS—H 1:29–30).

Nevertheless, we must remember that “signs come by faith, not by the will of men, nor as they please, but by the will of God” (D&C 63:10; see also D&C 24:13). And the Lord has admonished us that we “receive no witness [or sign] until after the trial of [our] faith” (Ether 12:6).

So, if we want to know if someone called to be an apostle really is an apostle, we don't look for miracles or signs. We look for personal confirmation from the Holy Ghost.

In line with the answer you got from BH.SE, that it's personal conversion that is the sign:

https://www.churchofjesuschrist.org/study/liahona/2019/07/we-are-witnesses-the-twelve-apostles-today?lang=eng

Today the Apostles’ commission hasn’t changed. “First and foremost, all the time, we are witnesses of the living reality of the Lord Jesus Christ,” said Elder David A. Bednar. “We are not administrators; we are ministers of the gospel of Jesus Christ.”

Apostles are commissioned “to be traveling witnesses” who go to “all the world,” said Elder Jeffrey R. Holland. “We want even the most distant unit of this Church, geographically speaking, to feel that there is a very close link between them and the prophet of the Lord,” he said. [...]

President Ballard’s duties have taken him to most of the countries in the world, allowing him to minister face to face to countless members and missionaries. Millions have tuned in to hear his general conference and devotional talks. But while he has a global responsibility, the Holy Ghost allows him to connect with and bless individuals. This seeming paradox is the Savior’s way, he said. “I sometimes get a letter from someone saying, ‘I was in a meeting, and you said something that changed my life.’ That’s the power of the Holy Ghost. The Lord micromanages His Church.”

A “countless number of sweet, simple experiences with members of the Church all over the world” defines the apostolic ministry, said Elder Bednar. “The Lord sends a member of the Quorum of the Twelve to specific places at particular times where we encounter faithful Latter-day Saints and others who often are struggling or are in need of comfort and reassurance. God orchestrates those interactions,” said Elder Bednar.

Sometimes we get to know these experiences when someone talks about them, but most of these we will simply never know.

Miracles happen, notably the first time Joseph Smith healed someone was an important church history moment because it lead to a lot of conversions where it happened. Of the miracles happening today, they do happen, but we only hear of a small fraction (because our religious culture is such that we carefully consider if, where and when to share sacred experiences such as miracles, apparitions etc.), but the biggest miracles following the apostles is the personal conversions coming through the Holy Ghost.