How can I look up to see if I have any unclaimed 401k money?

You mean from working?  Ask your employers or the 401K plans.  You should be getting account statements at least once a year or more.

Do you mean you want to verify that your reported your 401k distribution, on your tax return?  Look at lines 4a and 4b of form 1040. Prior to 2018 it would be on lines 16a and 16b of form 1040 (or lines 12a & b of 1040A).