Basically we should make a sharp distinction between two basic situation.
First situation is the initial diagnosis of the disease: in that case there is no specific RT-PCR or IHC technique which can help us. Stool blood as a screening or/than colonoscopy are standard approaches.
The second situation is the relapse. Beside the methodology clearly described in follow-up guidelines, there are no really established investigation to detect relapse. In some cases the detection of circulating tumor cells or tumor DNA could help, but these methods are not recommended yet for screening purposes.