Are there any tests that can diagnose a ventral hernia?

Yes. The best test to diagnose a ventral hernia is a physical exam by a hernia surgeon. Rarely, physical examination may be difficult; a cat scan of the abdomen can be helpful in those circumstances.
Yes. History and physical exam are the best ways to diagnose a hernia. It is also possible to utilize dynamic ultrasound and ct scan with and without bearing down (valslava) for small, occult hernias.