They were all 8mm but there is an 8mm upgrade that is important if you want to use modern 8mm ammo in it. Look for a rounded out spot where the bolt goes into the receiver which allows the longer bullets. Should also be an S on the top of the receiver but not always there. You should make sure there is a cover on the bottom of the magazine.
There are two major versions of this rifle also the true 88 version which has the metal barrel shroud and then a "mauser" conversion that makes the rifle resemble a mauser rifle other than the magazine. You have to decide which you prefer but I think most collectors want the original 88 version.
Other than those couple of things, just the basics. Bore condition, exterior rust, check especially close along the wood line. Check the stock for major cracks. Will most likely have turkish markings on the rear sight. Some cresent moons here and there, not always in the same place but the bolt and receiver are common. Bolt probably won't match serial numbers but is a plus if it does. Not sure what the going rate on these is, generally less than a non-turk marked
German 88. Look at the sold listings on gunbroker for an idea. Last I paid attention they were in the $300+ range if decent condition. Wouldn't surprise me if they were around $400 now.
I did check gunbroker and by the looks of things $300 is still the going rate. Now there is an exceptional example on there that sold for $600 but it still has the original finish and lack of finish on the bolt and receiver. Very nice. Very few are in that good condition.