Erectile dysfunction is diagnosed by a combination of individual awareness of your own health plus input from your doctor or health professional.
You will be aware of “normal” sexual functioning for you, and are the best person to judge if you feel it is declining.

Your issue may not be erectile dysfunction – an inability to get an erection, but may be low sex drive – that is not wanting sex, or premature ejaculation – coming too soon.
A doctor or sex therapist can help determine what the problem is.

If your inability to get an erection is just occasional it would not be considered “erectile dysfunction” and a few simple changes – drinking less alcohol, getting more rest, reducing stress – may right the problem.

But if it becomes a recurring problem affecting normal sex it’s best to ask more questions and your doctor can help here.

Diagnosis of physical symptoms
ED can be an early warning of other health problems like poor circulation or low testosterone so a general health check will help determine if there are physical causes for erectile dysfunction.

Blood tests will measure glucose (blood sugar) levels for diabetes, cholesterol and testosterone. Other tests on arteries and nerves will probably not be needed as they do not change the treatment.

Your doctor may ask about common medications you may be taking – for example anti-depressants, or beta blockers – which can affect sexual performance. If you suspect this is a problem ask for an alternative medication.

Diagnosis of non-physical symptoms
If your physical health checks out you may benefit from seeing a sex therapist to discuss life issues – your level of anxiety or stress, and possible relationship issues.

ED is usually non-physical in origin if you have normal morning and nocturnal erections, can masturbate to a firm erection, and your penis behaves normally when you are not trying to have sex.

A therapist may also help if your erection problems are more to do with relationship issues, performance anxiety, premature ejaculation, or difficulty in achieving an orgasm.
For more information on diagnosing erectile dysfunction see Do You Have Erectile Dysfunction Symptoms?