What test do they run to see if a pregnant womean has a virus (stomach) im 6mnths?

Rarely done. We rarely run tests to verify that someone has a virus. Notable exceptions would be test for HIV and various influenza viruses. In general, stomach viruses are nothing to worry about and will take care of themselves in a week or so. Just work at staying well hydrated. The brat diet (bananas, rice, applesauce, toast) is a good place to start until your stomach gets back on its feet.