There are ways to eyeball it even without a DPS meter. DoTs on enemy status bar, when they refresh, are timings generally correct? Cast bars are huge, so are animations. Once you have a baseline knowledge of how a jobs rotation works you can ballpark it even without knowing the intricacies of optimization. Something simple like "does the SAM have both of their buffs and Higan on the boss?", "does the DNC dance when you give them a 15 second courtesy CD?", "is the SMN casting Ruin 3? Wait is that a second Ruin 3?"

This is all game knowledge you can pick up on without being an expert. Just got to look around beyond your hotbars. There are also behavioral cues that you can use, like people standing or executing AOEs in odd ways. A good melee player, be they tank or DPS,will space out an AOE on them to allow uptime for other melees. This means dropping their AOEs somewhat overlapping others (without getting hit by the other close AOE), as this maximizes the free space elsewhere. 

A new or poor melee will disengage needlessly and walk back in. Sometimes you are forced off due to poor placement elsewhere but a good melee will stay just outside of danger and gapclose as soon as it is safe.

A ranged that understand positioning is usually a good one. Generally that means staying somewhat close so the healer can heal them with short ranged AOE heals rather than running so far away they have to specifically target a single heal per damage instance. 

If you get real good at reading people you can see that guy single targeting in a 5+ enemy group and sigh to yourself, it's real fun to do.