If they are white opaque - they are very likely to be Aquila Nylguts
If they are red - quite likely to be Aquila reds
If they are kind of pearly dark grey - likely Aquila Lava
If they are dark brown - likely Worth Browns
If they are a purplish hue - likely a Titanium Nylon, either D'Addario or GHS
If clear and high tension - probably a fluorocarbon, and could be Worth, Oasis, GHS, D'Addario, Martin, etc.. No "clear" way to tell
If clear/hazy white and low tension - probably a nylon, could be LaBella or D'Addario
If a hazy white and super smooth - probably a rectified/ground nylon, either LaBella or GHS
If VERY black and low tension - most likely traditional black nylon, which nearly everyone - LaBella, D'Addario, GHS, Hilo, etc etc - offers