You shouldn't be able to tell how much genomic DNA is present just by a single Ct value because this will be derived from cDNA and genomic DNA with the same sequence. But you can do a control without reverse transcriptase, see what Ct value you get, and should be able to assume this is due to genomic DNA in the sample. 

Hi,

It largely depends on whether you are doing on a prokaryotic or eukaryotic system. If prokaryotic then its not possible. but if you are working with eukaryotic system then just find out whether your primer spans a intron.if so you should see two picks in the melt curve analysis for gDNA contamination.

Hi Laurence,

Thanks for sharing your expertise on recognizing gDNA contamination in RNA samples. It is really helpful.