Optimal RNA quality is indicated if the upper rDNA band has twice the intensity of the lower rDNA band. Use a denaturating gel for this experiment. Regarding your question, you need to use a RNA size standard. Using a DNA size standard is not giving a reliable result for this kind of experiment.
If your aim is to just check the RNA integrity, markers are actually not needed. Resolve the samples under denaturing conditions (formaldehyde agarose gel) and check the ratio of intensity of upper rRNA band to that of lower rRNA band. The ratio should be near to 2.