There are appraisal services, but they charge for their expertise and certification. If there are not too many guns, how about taking a good picture of each side, crop and reduce the pixel count to something we poor dial-up users can download in less than half a day, and let us take a look. If they are in the stratosphere price wise, then a pro appraiser would be better.
As a general comment, condition is all. The amount of original finish, whether the gun is intact or parts are broken/missing, whether it has been altered (like a musket with the barrel and stock cut down), all figure into value. So does any documented connection with a prominent person (a gun known to have been carried by Bobby Lee is worth a lot more than of unknown history). Age alone means little in regard to value.
Value enters into insurance as well. For average guns, unless there are a lot of them, your normal home insurance should cover them; if not, a policy rider would be best, or even keeping them in a bank vault. Talk to your insurance agent. There are insurance plans specifically for high-dollar collectibles, but they are pricey, premium-wise.