Treat an aliquote of your viral stock with protease+ DNAse (Rnase-free preparations) and another with protease+ RNAse ( DNAase free preparations); purify or precipitate the expected RNA or DNA and run them on gel hoping to see intact RNA in your first sample or intact DNA in your second sample. There are options to inhibit RNA or DNA synthesis  too and ask the question if your virus propagates when DNA synthesis is inhibited ( Yes will suggest RNA genome). Probably it is most straightforward to do the experiment in the context of plaque assay.

Treat an aliquote of your viral stock with protease+ DNAse (Rnase-free preparations) and another with protease+ RNAse ( DNAase free preparations); purify or precipitate the expected RNA or DNA and run them on gel hoping to see intact RNA in your first sample or intact DNA in your second sample. There are options to inhibit RNA or DNA synthesis  too and ask the question if your virus propagates when DNA synthesis is inhibited ( Yes will suggest RNA genome). Probably it is most straightforward to do the experiment in the context of plaque assay.

I presume you can detect them using plaque assay. If you dont get any plaques in the presence of RNAse in your agar, then it will be indicative of an RNA genome. But of course this is a quick and dirty experiment!

Usually bacteriophage give clear plaque have DNA as a genetic material but if plaque is turbid than the chances of genetic material may be RNA, further genome analysis may confirmed the  presence of DNA or RNA in your phage   

Hi. I agree with Sadhana Sagar answer