As most people who are at all serious about the game know by now, limping is bad. There are almost no situations where it makes any sense at all. Having the initiative in the hand is just much more +EV in nearly all spots. So if you see someone limping, especially open limping from MP, LP or the SB, there is no question that this is a weaker player. Some regs will still open limp from EP or perhaps overlimp or complete the SB with a speculative hand. I don't always note that. I am looking for chronic open limping especially from positions where it never makes any sense to do so.
2) Posting a Blind Out of Position
This really is a dead tell. Basically every reg in the world knows these days that you should wait until the big blind comes to you before posting your blind. Since poker is a long term game and you can think of every session as just a continuation from the last one it simply makes no sense to pay for blinds more often than you need to. Some regs may post in the CO in a full ring game especially if there are known fish at the table. I do this myself sometimes. But if you see someone posting their blind from ANY other position it is without question a bad player.
3) Buying in For Less than 100bb
Since Pokerstars got rid of the ability to buyin for 20bb a few years ago on its regular ring game tables and exploit the well known short stack strategy there simply is no reason to ever buyin for less than 100bb. There are some people who swear by some sort of "mid stack strategy" but in my opinion this is just leaving money on the table. There were some legitimate strategic advantages to playing with a 20bb stack such as being able to ship over regs who open or 3bet too light. This is not the case with 40bb, 60bb or any other weird stack size. These stack sizes require post flop play. And if you are going to play postflop why wouldn't you want the most ammunition on hand possible?
More specifically though as regards fish one of the easiest ways to spot them is a bizarre buyin with cents in it. Buying in for instance at an NL10 table for $6.43. It is clearly their whole bankroll and obviously no good player would ever have their whole bankroll on the table. I generally just mark anyone with a sub 100bb buyin as a fish straight away.
4) Under-betting the Pot Postflop
This is another pretty clear tell to a weaker player. Especially when they are betting amounts such as a 1/4 of the pot or less. No good player would ever do that because it gives the other player(s) almost no incentive to fold at all and clear odds with basically any draw. Even bets of 1/3 have very little applicability in a NLHE cash game. Smaller bets certainly have merit in MTT's due to shallower stack sizes but in a cash game you should be betting at least 1/2 of the pot in almost all circumstances.
5) Mini-Raising Pre or Postflop
A 3bb open is pretty much the standard in online cash games everywhere now. There are some people who swear by mini-raising from LP and it has some merits. But anyone who is chronically mini-raising from other positions is almost always a weaker player.
The same goes for mini-raising postflop. There are some arguments in favor of it in certain spots but I think at the micros (especially NL2 and NL5) you should be raising for value most of the time. Mini-raising is therefore just leaving money on the table.
6) One Tabling
I know I said the "Top 5" but here is one more way that really should have been included. Credit to Willian Mates in the comments. Most poker sites give you the ability to search a player and find out what other tables (if any) that they are playing. Fish will be much more likely than regs to be playing just a few tables and often just one. It is not easy to 24 table when your VPIP is 78!
It should be noted that by tagging people immediately as fish if you notice them taking any of the above actions you will find out later that several of them are actually nits. This is because many regs at the microstakes will do some of the things above also. This is fine, just change the tag from fish to bad reg once you notice the low VPIP. Very few good regs would ever do any of the above and it is good to know who the bad regs are because we can make lots of money off of them as well.