The best way to know what type of data is in a file is to find out from the person or persons who created the file. In your case, since you got the file from the U.S. patent office, you should search the FAQs on their site to find out the format of the discs they supply and/or download and use the software that they supply to search the discs.

The second best way is to use the kind of format-snooping programs recommended by dorward.

The third best way is to use an online file format resource like this file extensions list. That list shows .dbo as a compiled dBase IV format. The problem with lists that name extensions is that more than one program can create extensions so you can't always trust the answer.

Unrelated to your question: do you have a good reason for searching the patents database? Do you know that searching it might actually harm your ability to protect a potential patent? If your interest is in protecting some invention, do yourself a favor and talk to a patents attorney.