The ring gear will be stamped 10-41 on a 4.10. It is easy to see. But that assumes the pumpkin has been removed.
It it is still on the rear end, you can to a rough check by jacking up one or both of the rear wheels. You'll want to check for two things. First, check to see if it is an open differential or if it has the Detroit Locker. Once you know if it is a locker or not, you can turn one tire while counting the rotations of the driveshaft. The calculation is different if it is an open driveshaft or a locker. If you do a search, you'll easily find the details of how to do this.
If you find a 4.10 with a locker, you hit the jackpot! If you find a locker without the 4.10, consider buying the locker anyway because they are hard to find and expensive. You can add it to your 4.10 center section when you find one.
The problem with checking it by turning a tire is that the 4.56 and 4.10 ratios are close enough that it is hard to determine exactly which one it is based on driveshaft turns. It gives you an estimate. Since you already have a 4.56, experiment with it to know what you are working with. That way, if you see a different result when looking at another rear end, it will be more apparent. Still check the ring gear when you pull it just to be sure.
Good luck,
Matt
