extract the mRNA

use RT-PCR to make cDNA probes

Isolate(if you want) and Radio Label various cDNAs

southern blot you genomic DNA fragments

And see if your probe attaches

where ever you see labelled attach was certainly a expressed gene in that cell in that state

you can also make probes for promoter regions etc to see if there are probable gene segments

and I suppose you can use probes based on GC content which is specific for coding exons (but m not sure of this method but we do use it in computational gene prediction)

Thank you ........

you can even isolated the target protein and construct the sequence of the gene from the amino acid sequence and then follow the procedure stated my Mr. sunny.

identifying gene through sequencing target protein is a tedious procedure. sure hyperdizing labeled probes with genomic DNA is the most logic approach