Many gun shops have firearms experts with a wealth of knowledge and resource materials that would be willing to research the gun and provide you an appraisal for a fee, often with the hopes that you will use them to sell the gun on consignment, crediting your appraisal fee against the consignment fees.
As others have said here, GunBroker is also a great resource, but be sure to look for guns that have actually sold at auction and not just with one bid.
Glock and M&P Armorer
NRA Pistol Instructor
Illinois Concealed Carry Instructor
Glocks: 17, 17L, 22, 22C, 23, 24C, 27, 34