If you find someones check on the ground you should either tear it up or return it to a bank and report the check as found on the ground.
You cannot cash a check you found on the ground if it does not have your name on it or the check hasn't been signed over to you.
If you try to forge a name on the check that you found and try to cash it you can be charged with forgery of a check and could get arrested and or fined.
If the check isn't yours you cannot cash it whether you found it or not.