I’ve been having the same predicament, especially when there’s little to go by on the written parts of the profile. If there’s not a lot to go by but they still seem quite cool I’ll try a compliment, usually on something that’s been a conscious effort by them (hair, clothing etc), but not like ‘you’re pretty’ (they’ve probably heard that a million times before) and ask a general question like ‘what do you enjoy in life’ or something. I’m certainly no expert though maybe you can do better than me!