How to doctors know if you have interstitial cystitis? What is the range of cost for tests? I don't have insurance but have every symptom. I am fearful of costs but want relief!

Dx of exclusion . All other possible diagnoses have to be evaluated and excluded to arrive at a diagnosis of interstitial cystitis. There is no one test that will diagnosis it.
Interstitial cystitis. Most common symptom is painful urination, that gets better with small urination voiding and recurs with bladder filling. A simple urinary analysis is all that is needed for lab testing. Most times there's no need for treatment since this is not a sign of more severe disease. Avoiding certain foods can help including caffeine, alcohol, artificial sweeteners, and carbonated beverages.
Clinical judgement. Interstitial cystitis is a clinical diagnosis and does not require any partyicular tests . Tests are required only to rule out other conditions that have close presentation , like uti, cancers ... If there is any suspicion . Therefore at least in theory your costs would be only an office visit to a doctor.