Pretty much look out for the same things buying a mandolin as you would an acoustic guitar. Arch-tops will give a fatter,fruitier sound and are more common for bluegrass style picking
Materials used in construction
Good metal work including frets
Quality of machine heads
Finish around sound hole edges
Perfling( the slot cut in the body edge for the binding to sit in) – this gives a good indication of overall attention to detail and finish
Arch top mandolins should be fitted with an adjustable bridge for poncing about getting intonation set correctly.
hope that helps, I've seen a couple of Stagg mandolins, and for beginner's instrument and getting a feel for the instrument and what you want in the long run, they're perfectly fine but not great.