League of Legends (LoL) Question: How to spot duo queues?

Another way that is possible is if they have the same tag, it is unlikely that two people from the same club are not queued together.

OP.GG: You can look at recent played with on the left side, and then compare it to his/her ranked match history. Op.gg won't directly point out that people are queueing together, but most of the time you can tell just from stats and match history.

Clan tags.

I personally look for certain character synergies (maybe it's a stupid idea but it works for me). For example, I'd instantly recognize a Rakan and Xayah in bot lane.

Other than that there's no real way (besides op.gg) that can tell for real if 2 people are in a duo. You often have to guess / aussume.