The claim markers SHOULD have the claim owners information on them. Claim number as registered with BLM, claim name etc. Not everyone marks their claims correctly for a number of different reasons. There should also be a Monument of Discovery posted somewhere on the claim. While corners should match up with the claim boundries, the MOD can be located pretty much anywhere on the claim. A proper MOD will have the contact info for the claim holder as well as a map of the claim, date it was posted etc.
Now.... If you can't find any of the markers or they are not filled in correctly your next option would be to go to the county recorders office and try looking it up there. This will at least give you contact info, when it was filed and what the actual boundries of the claim are SUPPOSED to be. (These don't always match up with the markers for a number of reasons) The county recorders office will only be able to tell you when it was filed on and by who. They can not tell you if the claim is current. For that information you will need to contact the BLM or use the LM2000 on line. LM2000 is not always up to date so contacting the office is usually the best way to get the most current information on the claim(s) in question.
Once you've gotten the information, you should contact the claim owner to make sure that they have no problem with panning. That you're at least attempting to find out the status is commendable. So many people would just go for it without asking.