Try washing out excess of HCHO and imaging it with UV light against a thin layer chromatography sheet, with or without adding ethidium bromide stain. Of course you have to be careful about not contaminating it with RNase. use DEP-treated buffers throughout. Visibilty will depend on how much RNA you have loaded on the gel.

@Deepak Bastia

Washing out formalin with DEPC-water? Would 30min be sufficient? I am a little confused about the TLC sheet.  Do you mean place the gel on top of the sheet on the light box? 

I suggest mix Etbr with your RNA sample just before loading to the gel at low concentration, which help stain only RNA and less background staining. Also all things/ consumables  used for RNA work should be cleaned with DEPC or Chloroform. Gel tank cleaned by 3% H2O2 solution overnight.

@Soumya

Yes, the tank was cleaned with SDS, then rinsed with ethanol, then soaked in H2O2 for 10min, then rinsed clean with DEPC water. 

I was trying to avoid staining before the run because intercalation increases relative molecular size and decreases mobility.  For a qualitative analysis this is fine, but I was trying to run a gel to determine RNA fragment sizes., and therefore need to stain the gel after the run.