Gauging job performance - how to measure them all?

[Lyrinn]

If you can keep everyone alive while clearing content, you're good to go. Simple as that.
If you can keep the party together and alive even with people making mistakes and standing in bad things, even better.
If you can DPS in-between heals while doing all that with good MP management, you're ready for the coils.
If you really want to push yourself, try solo healing anything that isn't FCoB without a bard playing ballad.

Edit: For tank,
If you're holding hate on everything you're supposed to and not losing it, you have basics #1 down.
If you're using cooldowns appropriately (which means different things depending on content), you have basics #2 down.
If you're positioning mobs/bosses properly (i.e. not facing AoEs at party or blocking flank/rear from melee DPS), you have basics #3 down.
If you can do all 3 of those, you're a good tank.
If you can snap aggro on top of all that, you're ready for the coils.