So the main thing here is that you're going to have to use out of game resources to find a decent guild. Any guild worth its salt will be advertising in channels other than just trade chat. You probably don't want to be in just a social/casual guild as those are the types of places that will go dead before too long, and only revive (if they revive at all) when new content drops. The best bet is probably to become an adjunct member of a more serious mythic raiding guild, even if you're not interested in raiding yourself.
To do this, first I would figure out what server you want to be on - or at least limit yourself to a few choices. Personally I wouldn't go on anything less than a high-pop and/or high-progression server -- you'll have a better economy and more opportunities. Definitely not any "connected" realms. There's about a dozen at most servers that fit the bill right now, check
on WoWProgress.
For guild ads from all realms, look
here. You can also check the MMO-Champion
recruitment forum or
/r/wowguilds. Otherwise you look at the realm forums for any realm(s) you've narrowed your search down to and there should be plenty of recruitment posts there as well.
An organized guild should also have some form of website on which you'd put up a formal application, even as a casual member. Higher quality guilds will generally have custom websites (e.g.
my guild's) vs. a premade enjin/shivtr/whatever.
Other than that, you should have a voice interview with a recruitment officer or the GM before transferring or otherwise joining up - even better would be to sit in on a raid night to see what the overall atmosphere is like.
I think that's about all I can think of right now.
