Are there tests that can easily detect gerd?

Yes. The tests to accurately diagnose reflux include an egd (upper endoscopy), a 24hr ph study, and an esophageal manometry. These will accurately diagnose gerd while ruling out problems that mimick reflux.
Rarely needed. GERD is a pattern affliction that most people will have to some degree, even if just transiently. The development of drugs that reduce the acid activity of the stomach have made their use a pretty standard office tool when this is suspected. A good history & exam followed by response to a med like Prilosec within 3 weeks can reduce invasive testing to resistant cases.
Yes. Both endoscopy and pH testing are fairly effective in detecting gerd. Respone to Prilosec or similar is also a fairly good (and much cheaper) test.