1. Your action bars have some very distinctive holes in them.
I'm not talking about missing skills that you're used to having on your main, but about finding holes where you know you used to have abilities on this character. Blizzard rarely removes spells, but even when they rework one (which they do reasonably often) it often gets taken off your bars and you'll have to manually put it back. A holey action bar is usually a good indicator that the class has probably gone through not one, but several revamps since you last played it.
2. You're missing skills that you really should have.
For example you might take your level fifty-two warrior to the warrior trainer just in case and find that she offers you an ability that's trainable at level six these days. But you never had it. Awkward.
Even better is logging onto a character in her thirties and realising that you don't have the basic riding skill or a mount, because you haven't played her since level twenty mounts were introduced. Bonus points if you're on the other end of the world from your race's riding trainer and have a long walk ahead of you while people ten levels below you zoom past you on their spiffy mounts.
3. You have lots of glyph slots and they are all empty.
Meh, last time I played this character, glyphs didn't even exist! Depending on your class it can take you a long time to actually notice this one from my experience, unless you have a frequently used ability that gets changed noticeably by a glyph (like a resto druid who is suddenly surprised to see swiftmend actually consume a HoT again).
4. You have a completely inappropriate seasonal buff on you.
A guildie recently helped me out on one of his rogue alts... and he was a turkey, in late summer. Most of my own Alliance alts have had fire festival fortitude on them for years now.
5. You have quests in your quest log that don't even exist anymore.
Daily battleground quests, daily Northrend dungeon quests, you name it. One of my Alliance alts had one of the last steps of the old Onyxia attunement chain in her log for ages.
6. When you look at your achievement panel, it tells you that you did a lot of impossible stuff today.
So, you've only just logged on, but apparently you've also run Uldaman, acquired a coin of ancestry and opened a Winter Veil present. All in October. Yes, you haven't logged on in so long that the achievement system introduced in WOTLK is only now catching up with all the stuff that it knows you did before its introduction. Neat!
When was the last time you reactivated a character that had been abandoned for that long?
18 minutes ago