How to deduce if RNA virus infection or DNA virus infection
If you mean simply by looking at symptoms, then there is no way, especially if in the context of a medical visit.  You're better off learning the usual suspects and figuring out the patients risk and likelihood for a given infection.  A good example is Hepatitis.  Hepatitis B Virus (HBV) and Hepatitis C Virus (HCV) have similar (enough) outward symptoms, but the former is a DNA virus and the latter an RNA virus.  In fact, looking at enWikipedia's list of acute causes, it's all over the place!

If you can actually assay for it in the lab, then of course.  Although...