Make sure if selling it that it is still on face.
I would say 80% of the second-hand guns ive seen in dealers rooms were well off face which is an accident waiting to happen.
The simplest method is to remove the forend, grasp the actual action not the stock hold it so one side of the action left or right it dosent matter and both barrels are facing you and then give it a gentle shake if you feel any sort of minute wiggle its off face, you can also see by how far the lever is to centre but this is not always an accurate representation as the lever can at times be close to centre and still have plenty of engagment on the bolt etc.. You can also check with a feeler gauge.
A list if things i would look at when valuing a gun would be this

Is the gun on face?

Are the solder joints all still properly joined ( you do this by holding the lump and tapping the ribs with a piece of copper/brass etc if they ring like a bell the whole way down then its fine, if there is a thud anywhere the barrels need to be resoldered. Which is a big job and should only be completed by somone who really knows what they are doing.

The inner condition of the bore and a measurement of the wall thickness.
Then i would look at things like the grade of wood,
hairline cracks in the stock,
the overall condition of the wood and of course metal etc.
The overall condition of the bluing,
ejector timing
if the barrels are loose due to forend wear etc.

If i were you id simply bring it to any dealer that has something you want to trade it in against and see what they say because most of them are only going to look at it and give you a price based on how it looks aesthetically rather than from an actual wear/safety aspect.