My xwife has recently filed 4 medical support for her oldest son who is not mine. She states in the divorce papers (3yrs ago) that there is no childern from our realationship now supposably she claims through dss that I signed something stateing that I am. How do I prove I'm not

Ask for DNA test to prove it.

... now supposably she claims thru dss that i signed something ...

It appears you are reacting to rumor based upon hearsay.

Ask to see a copy of this document you supposedly signed.

And, eventually, you will want to notify this "support court", whatever that is, that the divorce court has already determined the issue.