Just moved in - how to tell quality of in-ceiling speakers?

I think you are going about this the wrong way.

The quality of your speakers should not dictate the quality of sound you want to have.

Having said that, for ceiling, stereo speakers I’d go with convenience over audiophile cred. Something that is fully network aware, and has a phone app. Again, being ceiling speakers, your style of listening may tend to be more background/mood music so having tone controls that let you set the sound quality and level appropriately is something you should consider. 

Another thing to consider might be whether you also have outdoor speakers you’ll want to switch in/out via a network app.

Would be something if they were BG or something like that. No way to remove a grill or something to look?

BG use to have surface mounts with 6-8-10" WCF low/mids and a Neo 3 for the mids/highs. I heard a shop with 8 8" units in the walls. 2 units per, one was upside down. So 4, BM T T  MB, in the short wall with 2x6 studs..

GREAT shop sound for darn sure.. He gave me 4 brand new 8" and 4 10" units.. for a plinth. Still in the BG boxes. He was going to add them ALL to his shop.. 8 was more than enough.. I think, lucky me? We'll see.

Surround speakers are generally a waste in walls. Only you can control the $$ amount of waste.

A family member spent thousands to have surround speakers fitted in the walls only to have the electrician install junk.

It’s unlikely that they’re spectacular,

My vote for understatement of the year. So far. 

Use a 9volt battery to make them “thump” if it’s a clean thump I’ll bet they work ok.

Thanks for the suggestion, that seems like the best option. It’s unlikely that they’re spectacular, but if they are it would be a shame to miss out!

You should have a $25 amp for backup and test purposes. Plug your phone into it for a source. You will spend more than that @ minimum wage trying to find out what they are.
And that is meaningless if they are fried.
If you don’t have and can’t borrow, buy on approval.
If they previous owner didn’t tout them they are probably crap.