Bare minimum, a healer should be able to keep people alive for an average pull (1-2 mobs), and immediately cleanse debuffs that significantly impact gameplay (e.g. paralysis, slow, heavy on mobility-reliant classes, major poisoning that's difficult to heal through, etc.). And preferably understand how initial aggro works, so they don't do stuff that interferes with tanks' pulling and endanger themselves.
Beyond that, I gauge healer performance by how they use their time, and how much they demonstrate team play.

For example, I've had...
- WHMs who Cure2-only
- Healers who massively overheal by just spamming heals even if the tank is only down by a tenth of their health
- Healers who Stoneskin everyone throughout the fight
- SCHs with Eos on auto (goodbye Whispering Dawn)
I won't hold any of this against them if no one died and they didn't run out of MP. But under most circumstances, it's not going to earn any comms from me.

I've had healers who are obviously not team players.
- run ahead to pull more mobs, especially in a way that causes a lot of chaos
- don't ensure the tank is Stoneskin/Adlo'd before a pull, thereby forcing some tanks to only pull single groups at a time, when everyone is obviously capable and willing to do more
- don't take care of boss mechanics when they're in the best position to
- run off into space or orbit when they have aggro of something that doesn't hurt -that- much, thereby preventing anyone from grabbing the monster[s] off of them
- Fluid Aura anything that's affected by knockback, thereby knocking something out of a mob, away from melee, or out of a caster's line-of-sight and interrupting their spell

As someone who does instances a lot, I really really appreciate it when healers help make the run faster and smoother, where possible. This doesn't necessarily mean DPS'ing.
Most tanks don't require your immediate attention every second, so you have plenty of time standing around. Especially if you make good use of HoTs as a WHM, or keep in mind how much your fairy can handle as a SCH. If you can DPS, great. If you handle boss mechanics so DPS can maintain their better-than-a-healer's DPS, even better.


As for how do I gauge tanks and DPS..

Tanks:
- Are they fast, or do they spend a lot of time walking, standing around, milling about?
- Do they properly hold aggro? (e.g. A tank with an i100 weapon should still be able to hold aggro off of someone with an i115 weapon. A tank with an i90 weapon, or atma weapon, may have trouble with aggro at first, and I can forgive that.)
- When making large pulls, do they tag everything along the way so that all names are in red? And do they pull smoothly instead of starting/stopping along the way?
- Since a lot of tanks could use advice.. how do they respond when given advice?
- Do they have good judgment and know when to not overpull?

DPS is mostly judged on a per-class basis. Like if I see a MNK not bothering with positioning, then I know they're doing it wrong. Or if a BRD never sings, or improperly sings so all they're doing the whole time is nerfing their own damage without helping anyone. That SMN who keeps leaving their egi behind, or who Ruin-spams only? Oh hell no. If I can tell that a DPS is obviously a team player, regardless of how high or low their DPS may be, they'll probably get my comm. Like if I see certain skills getting stunned or silenced and I know it was them.
I think the only thing I can apply to all DPS is.. if the tank has a mob of 6+, are you AoE'ing? And a token ground-DoT does not count if they're single-targeting otherwise. A lot of DPS don't earn my comm on this point alone.