If they use their toolkit, as with all roles.
That means using partybuffs on cooldown and not just on bosses, using selfheal when they ate something, dodged an aoe heal or are in danger of dying, Peloton between pulls and Samba/ Tactician/ Troubadour during big pulls or partywides on phys ranged, aoeing in big pulls and keeping self buffs up if applicable, ressing on RDM/ SMN if necessary. I don't expect stellar dps and perfect uptime, I just want to see someone trying to utilize buttons that are not straightforward 1-2-3 dps skills.
I'm not a fan of "comm your tank/ healer" rule because of some nonsense reason like "they have sooo much responsibility, show them love ♥". I comm someone that uses their toolkit. Spamming Medica II, forgetting mitigation or doing single target in big pulls isn't using the toolkit.
All roles have an equal chance to get a comm from me, no brownie points just for chosing a specific role.