Honestly?
Ask him!!
Hon - What kind of monitor were you hoping to get for your next upgrade?
1080P 1440p 4k? (This is the resolution)
Size( 24 inch / 27 inch - Ultra Wide )?
Gsync / FreeSync?
What you need to know is the type of Video card he has... and how much RAM is has..
Example if he has a GTX 1080Ti with 11GB of Ram - That is a high end Video Card and you want a really good Monitor which can cost a lot of money..
but if he has say a RX 580 then you would get a completely different Monitor..
You can just say you were looking into something like this because your GF / Mother / Friend was talking about monitors and you had no idea, but you know he was looking into them.. and ask him what he would get?