I'm not a medical student, but I do conduct cancer research full-time. There's no rule regarding TSGs being written in upper-case vs. oncogenes being written in lower-case. In the primary literature, it just depends on the authors.
If I were in your place, I would just try to understand the protein's function in helping you determine whether it's a TSG or an oncogene rather than whether it's written in upper/lower case. Loss of function is associated with TSGs while gain of function (ex. activation, amplification, etc) are associated with oncogenes. If the question stem mentions something regarding that, it would be a pretty good giveaway regarding whether they're talking about a TSG or an oncogene.
Again, I'm not a med student, but I just wanted to point out that there's no rule regarding upper/lower case naming of TSGs and oncogenes. Hope this helps.
Edit: I've seen the trend that Tupacalipse mentions in the world of genetics, but cancer biologists don't really follow that unfortunately.