tests-for-erectile-dysfuntion

Tests:

Blood tests, including complete blood count, metabolic panel, hormone profile, and PSA
Neurological (nerve) testing
Nocturnal penile tumescence (NPT) to check for normal nighttime erections
Penile ultrasound to check for blood vessel or blood flow problems
Psychometric testing
Rigidity monitoring
Urine analysis

The Sex Therapist is best person to decide which tests to do.
Most experienced Sex Therapists do not require the above tests for diagnosis or treatment.

Source
http://www.nlm.nih.gov/medlineplus/ency/article/003164.htm