Check the serial numbers..... You can print them from computer. Some are worth way more than others. Most go for around $400-500 range but some are in the $1000-1200 range.
I bought a 16ga model 12 several years ago for $300 it was made in 1937 & is emaculant. It in the $1000 range
Check Gun Broker, Guns International, Guns America these sites will also give you an idea at what you're looking at & what it's worth by checking serial #s