He's dancing with Grandma
The Crasher's theory: He'll dance with an old lady -- whose memory will be so foggy, she'll assume he's a family friend or even her grandson -- and this will have the twin effects of: a) scoring instant credibility; b) scoring a bridesmaid. Don't trust this guy.
His stories keep changing
The go-to Crasher trick: If he's talking to someone from the groom's side, he'll say that he knows the bride; if he's talking to the bride's side, he'll say that he knows the groom. It's sneaky; it's effective. Unless...what if the bride and groom confront him together? Checkmate.
Every Crasher sees Vince Vaughn as his hero, and the ultimate Crasher fantasy is to bro-hug the groom, dance with the bride, cut the cake and then give a rousing speech. At 99.999 percent of weddings, people you barely know simply don't really act like that. So if someone does, keep your guard up.
He came in late or pushed through the receiving line
Crashers make their entrance at one of two times: 1) boldly jumping into the receiving line, where people are too distracted to say anything; or 2) joining the reception after the speeches, when the party becomes a free-for-all. Keep on alert.
He's a great faker
The Crasher will have a premade alibi like, "I went to high school with Larry," and then back it up with, "You know, Laarrry, that craaazzzy ol' such-and-such!" and then good-naturedly clasp your shoulder, coasting on his charm.
He's never sitting down
Sure, this guy is a good faker, but here's what's harder to fake: the table he's sitting at. If the wedding has assigned seats, the jig's up. Unless, of course, he just never sits down. Time for dinner? He's in the bathroom, at the bar, anywhere but at the tables.
He seems to be dodging the wedding planner
Your planner knows that guest list by heart, and she's seen her fair share of weddings and knows how to spot a Crasher too.
He says he's a fifth cousin (seriously, who would invite their fifth cousin?)
He probably really isn't Philipo, the groom's fifth cousin who's been trout fishing in Alaska. C'mon, you knew that was a dead giveaway, right?
He wasn't at the ceremony
The Crasher's here for the party, and there's no chance that he wasted his time at your ceremony. (No offense.) So ask him questions about the actual ceremony, like, "Did you see the blimp that passed by overhead? So cool, right?" An actual guest would be confused; a Crasher would play along, saying how much he loved the blimp.
He's a really good dancer
What's the point of going to a wedding if you're not dancing, drinking and eating? Now imagine spending all of your weekends at weddings (that's what Crashers do). You're going to pick up a few moves -- and you're not going to be shy about jumping on the dance floor when you hear "Sexy and I Know It." Seriously, even if you're not. Remember, it takes a certain confidence to crash a wedding, after all.
Now, in my defense, what's the real harm of a Crasher? Even if you catch a crasher, what, exactly, are you going to do with him? Have a confrontation? Call the cops? Beat him up? That hassle is (probably) greater than the downside...of losing some cake and booze that you've already paid for. If someone crashes your wedding, take it as a compliment -- this means you planned a hell of a party with good food and attractive people. Now you have a funny story that makes your wedding even more memorable.
Well done. Besides, it's really (I mean, really, really) unlikely anyone is going to crash anyway.
When not crashing weddings, Jeff Wilser is the editor of groom website ThePlunge.com. You can follow him on Twitter at @JeffWilser.