You can use Reverse transcriptase assays, they exist for example for HIV but you probably can easily make your own. Basically if you know the original target for cDNA generation, add it in abundance. You can have it expressed depending on its length, maybe take a shorter piece of the 3' region where the RT activity starts. Express it from a pol III promoter like H1 or U6 to have a defined RNA fragment (make sure you avoid TTTT sequences in the sequence as these are the transcription terminators for the pol III polymerases). Include a specific region/mutation so you know this is "yours" and not the original virus template. Then harvest the nucleic acids from the cell and treat with RNAse. If your previously abundandly produced RNA is now cDNA, you know it was reverse transcribed.

OK Ulrike Jung, I will go for it and see. Thanks!!!