Do Christians have "secret" (handshakes for example) ways of knowing who other Christians were?

Yes.

It is very easy to shoehorn in an obscure Bible verse into a conversation, without attribution. And it is easy to tell if another person recognized that verse (because if they either smile or acknowledge it openly or respond with another biblical phrase). Can be in a college lecture or study group, with a Christian uni professor low-key sermonizing the class. Or in the workplace, a stranger at a coffee shop type of conversation, or a new neighbor who just moved in. Or a match from a dating site.

There are other ways, for example, name-dropping a lesser-known Christian rock band that doesn't have an overtly religious name. ("What music do you like?") Or asking about a person's network of acquaintances: "Do you know Mr. Davids or Mr. Lee or Mrs. O'Nell?" where Davids, Lee, or O'Nell are the lead pastors, associate pastors, youth-group pastors, or pastor's wives of the various churches in the area, or a mega-church of a nearby big city. Everybody knows the pastor's wife; she is always on the phone with people's problems.

And then there is the dreaded "Christianese". The (ugh) "fellowship" rather than more colloquial "hang-out", "blessed" rather than "lucky" or "happy", the semi-insulting "I'll pray for him/you" rather than the more succinct "bless your heart" of Southern provenance. An insightful idea becomes a "word of knowledge", a likely prediction becomes a "prophecy", and constructive criticism becomes a "word of correction". Agreeable personality or good character becomes "fruit". The Bible, the Sunday Sermon, or God Himself becomes "the Word". A sense of purpose or moral resolve to help out in some way becomes a "calling". "I felt called." "You are called." Good personal judgment becomes "discernment" or "discerning of spirits", which can be the supernatural ability of the Christian to determine the will of God, but also the practical street-smarts to avoid falling for scams and the like. Ordinary Christians, friends, family, acquaintances, even youtube characters are ascribed supernatural, super-hero identities: it is said, manner-of-factly, that so-and-so is a prophet. So-and-so is a priest, an apostle. That person's alleged angel sightings, or spiritual insights (which can be as simple as two memorized verses with one sentence of original commentary), are proof of this.

The church congregation or any two Christians together become "the Body". Not to be confused with unsatisfied libido, known as "the Flesh". (Unmarried Christians always "struggle" with "the Flesh".) And you also hear words like "outpouring", which "rains down". "Outpourings" that "rain down" don't refer to anything in particular. Same with "spiritual warfare"; prayer, meditation, affirmations, music, poetry, dance, exorcisms (where people spasm and scream, like in the movies), getting sick, getting well, having a difficult customer-service job, earning good grades, dealing with a lawsuit, firing an incompetent employee, disciplining disobedient children are all ways of "fighting" "spiritual warfare". Just listing what all counts as "spiritual warfare" in practice could be the subject of thousands of books, and it has. St. Paul would have reworded a couple verses in Ephesians if only he knew. Bless his heart.

Any and all disagreements with a non-churched person (or a person of unknown faith affiliation), if the disagreements are potentially important enough to have consequences or require a resolution, become "persecution". Christians who normally might use cuss words in casual conversation won't use them with another Christian, because of the "accountability" (another Christianese cliche). Greek and Hebrew words get dropped into conversation. "Shalom". "Parakleet", "logos", "agape". Then come the book-club recommendations and the dreaded "Would you like to visit our Wednesday-night home study group?" Or, worse yet, a multi-level marketing scheme. "Would you like to become a vessel for the Kingdom to financially bless others?"

Ironically, finding another practicing Christian out in the wild doesn't necessarily mean that the two Christians will get along or become good friends. Two random Christians from different churches or faith traditions might find that they don't have the same theology or practice; speaking "Christianese" with another random Christian can sometimes lead to conflicts of opinion. Things like opinions about the validity of healing-and-miracle ministries in the present day, Genesis literalism, or the likelihood of a biblical end-times scenario in one's own lifetime. The question of whether or not it is acceptable for a Christian to desire to become wealthy and successful (like Job! like Joseph! like Solomon!). Or, of course, whatever latest activist-involved kerfuffle that the latest big-name TV preacher has fallen into.

There is, of course, also the fact that two Christians might not form a friendship simply because one or both of them have an obvious personality flaw that make it difficult to get-along with. Unrelated to any Christian beliefs or practices.

If you aren't a practicing Christian, don't intentionally use any of these secret-handshakes. There's no prize for "catching" one of us out in the wild. And if you are a Christian, please, please stop.

Source: am American evangelical, from the relatively secular, pagan outpost of San Francisco, California.

The idea of the ICTHUS (or Christian fish) symbol was an early example of this. It wasn't pointless, as it was used to identify each other in a society where persecution of Christians was extremely common. One person would draw one arc of the fish, probably in the dirt, and another would draw the other if they were a Christian as well. This wasn't done just for fun, but for protection, in much the same way as how WWII Allied soldiers would whisper "flash!", and another Allied soldier would respond with "thunder!" in order to identify themselves.

The Sator square was likely used to secretly advertise a house as Christian to other Christians in the first century.