Hi. So firstly if you detect bands on top of gel when you are running an RNA sample, its probably DNA contamination. This is because DNA being larger in size travels less as compared to RNA. Now, if you are getting two consecutive sharp bands (28S and 18S) it means the sample contains RNA. During RNA contamination in DNA sample, mostly you will find smear in low molecular weight range. Another good way to detect practically RNA contamination is by running a normal agrose gel, take the image and then soak the gel in TE buffer. Then add a little amount of RNAse and keep it for shaking. After 5-15 minutes, visualise the gel and compare it with the former image. This time you may not find the low molecular weight smear. This will clearly indicate you about the RNA contamination in your sample (The latter method is been shared by Ali Mahmoudpour in one of the research gate question). I hope this helps you in finding your answer.