Staying on topic others have already given their two cents, and you have a better understanding of how tanks should work. Key thing is looking for buffs/cooldowns.

The biggest and most common one is Rampart - 90s cooldown reduces damage taken by 20% for 20 seconds, can typically use it every pull or every other pull depending. EVERY tank has this ability, it's a role ability, you can't miss it, one of the most useful and fairly short cooldowns a tank can have. Looks like a Blue brick wall buff with a red background (mostly blue on the bottom, red on the top).

Next we have Arm's Length, and while this does prevent knockback it also slows any enemy that hits you by 20%. 2 minute cooldown, lasts 15 seconds, typically won't work on bosses, but it's perfect for big trash pulls. Icon looks like a guy standing doing a falcon punch to the top left side of the buff. Another role ability, there isn't a tank who won't have this, plenty of tanks who won't have it on their bar though, or forget it's an actually useful cooldown.

Every other cooldown is tank based, but typically every tank will have a very short cooldown mitigation ability they can use, such as The Blackest Night for DRKs, or Raw Intuition for WARs. Typically short duration, but very fast cooldowns that can be popped multiple times during big pulls and bosses.

Then you have the beefier damage reduction cooldown, 30% damage reduction for 10s with a 2 minute cooldown. All tanks have one they're just called different things, Warriors is a little more fancy because it deals damage back for every hit they take while under the effect.

Honorable mentions will be the odd things some tanks get, examples would be Camouflage for Gunbreakers, increases parry rate and reduces damage, Dark Mind for DRKs which reduces magic damage taken by 20%, etc. All tanks get variants of useful cooldowns such as these that you may or may not see a version of on other tanks. Thrill of Battle is a good example for Warriors as it increases their max HP but also increases any healing done to them by 20%, sadly Convalescence used to do the healing buff and it was useable by all tanks but they took it out and only gave it to the warrior instead.

Finally you have the O-Sh*t button that grants invulnerability for a period of time, typically has a longer cooldown, and each tanks is different in how they work.

Back to the main topic, if you see a tank using any of these before a pull, not at all, or all at once, they're a bad tank. The more cooldowns you use at any given time is typically a bad thing due to diminishing returns on abilities that reduce damage, and once they're over, you've got nothing else to use. Much like most healers don't typically pop everything at once, we can, but it's not efficient, same goes for tanks.

Good tanks will typically use a single cooldown first, Rampart being the most common, and then cycle through their other cooldowns as necessary.

Great tanks will by vocal about their cooldowns, and I don't mean typical things like popping a damage reduction cooldown and having a macro for it (although it can be useful to some healers), but I'm talking about the ones who talk to their healers, ask questions. "Are you comfortable with big pulls?" "I plan to use my invuln for the next pull, just a heads up" "First tank buster I can heal myself through it". It's the little things of communication that help.

I go through both sides daily as a healer and as a tank. As a healer I typically let the tank know I can handle big pulls as long as they use cooldowns, if I'm having issues keeping up, or my mana is low and I have no cooldowns to help I will say something. As a tank, I will ask the healer first and foremost if they're ok with big pulls, why I'm only pulling a small pack of mobs due to having no cooldowns up for a big pull, etc. I always look at their mana before a pull and if I see them down 25% without a Lucid currently going I'll either wait a few moments or just ask if they're ok to keep going. I would rather go through a dungeon/raid/whatever a little slower if it means we can do it successfully without any wipes.