Instructions
Check the dates. While the exact beginning of his career with Colt and Smith & Wesson isn't known, Young worked for Colt from the 1850s through 1870s and for Smith & Wesson from the 1870s through the 1890s. If the gun in question does not match those dates, it most likely has not been engraved by Young.
Look for a signature. Young did not always sign his work, but it can help your case if you find a signature. Generally, he would sign the work in the location where the base of the gun meets the handle.
Authenticate the work. If an authorized gun appraiser identifies the work as one completed by Gustave Young, you can ask for a certificate of authenticity for your piece. This will give you credibility and allow you to collect a large sum for the work from a collector or at an auction.