Well, this could be a matter of opinion as well, but in general a single slab of solid spruce (Engelman or Sitka commonly) or cedar cut in 'bookmatched' fashion is considered to be a higher quality guitar top than a 'plywood' top.
'Bookmatching' - Usually a top has a joint right down the centre-line of the guitar. If you imagined taking each side of the top and folding them together along this axis - this is how the original peice of wood was cut - right through one slab to make two peices. The grain is nearly equal and opposite when you look on either side of the centre joint of the top, because those two surfaces are on either side of where the blade cut.
Really low cost guitars often use a plywood top rather than a solid sheet of wood, although some companies would have you believe this is an improvement over a more expensive solid top. It is possible that a ply-top is stronger, but that is not the ultimate goal of an instrument...its tone.
A solid top is what is used on nearly all high quality guitars (and many not so high quality). It ages nicely, yellows nicely, and if Im not mistaken, will 'mellow' in its tone with years of proper use and storage. Nearly all high quality stringed instruments (like violins, cellos, etc) use solid wood tops - usually spruce as well.
Seagull use solid tops, as do some of the Yamahas. Look at the edges of the sound hole on the top. If you see grain continue from the top,its solid. If it looks like plywood, then it is. If it's painted its hard to tell, could be either. You will have to ask specifically.
Having said all this, I supose its still the ears and hands that should do the deciding.
Hope this helps!