I often find that passcodes and the like are exceedingly ineffective. Even in games where I am just looking at it and looking over the sheets, it's really easy to notice a common trend, be it a passcode, saying your favorite colour, or whatever. It's not nearly as clever as most people would think, and there's a much easier way to sort through it all. It's exactly what Tick said; scrutinize the character sheet.

Now, this doesn't mean being so anal retentive like in Glitchy's case where minor spelling and grammar errors are the cause for rejection, it's requiring character sheets to be detailed and setting appropriate, and including things in it like how world-changing events effected your character, and keeping timeline appropriate. It's usually pretty clear if somebody took the time to read over the introductory OOC post and incorporated elements of the lore into their sheet compared to those who just got the gist of the game and whipped together a character sheet in ten minutes using only the most rudimentary of information. It's usually pretty obvious when somebody puts in a good amount of effort into their character sheet, which doesn't necessarily mean long, because they took care to address several of the details. The GM needs to be proactive about making sure the applicant's sheet is appropriate and matches the quality and standards that are set out and shouldn't be afraid to give it a detailed critique about why something doesn't work. If a player legitimately tries to address the issues and is more than willing to work with you the entire way, then they're probably a good fit for the game. If they're making a giant stink over changes you want to see or they half-ass in a justification for something, then it's probably a sign that person isn't going to work for your game. 

TL;DR If it's obvious somebody didn't read everything, then don't bother with them unless it was an honest oversight. It should be pretty clear if someone is putting in an effort. A password isn't going to do anything for you other than make it really easy for someone who takes five seconds to glance at a couple character sheets to figure out how to get "in" the game. It's counter-productive, and in my opinion, lazy GMing.

[quote=Brovo]
Oh, that reminds me, two other things I'll do.1) Check posting history. If they have a habit of jumping out of RP's, keep that in mind.2) Reject some people for (I can make up bullshit as well as the next guy) and wait 3-4 days to get back to that person. This tends to vaporize 50% of the problem players right there, since most of the impatient ones also tend to have fragile egos. Bonus points in that it also works against people who cannot stand criticism, which if you have a long lasting RP, will invariable occur to everyone at least once.
[/quote]

I don't really agree with 2, at all. Rejecting someone to see if they'd be grateful to be accepted back a few days later just seems like an unnecessary dick move out of Fight Club. Starting a relationship with a player by establishing yourself as a manipulative liar is a terrible way to go about it. Just spend time going over the character sheet and go into a hard, detailed review that picks apart anything that looks questionable. If they are open to criticism and work at properly addressing it, then you have a good player. If not, then you get it out right then and there. Don't play head games.