The diagnosis is generally suspected from the history.

Any cyclical symptom may indicate the presence of the disease. Increasing menstrual pain maybe an indication of the development of endometriosis.  The pain may be different from the more common form of menstrual pain which tends to decrease as one grows older. The pain may be present both in the middle and sides of the abdomen and may not respond well to over the counter pain medication.[i]

Lower abdominal pain just before the period, infertility and pain during intercourse may occur. Bowel and bladder pain particularly around the time of menstruation, more frequent need to empty the bladder and diarrhoea or constipation, particularly around the time of menstruation, may indicate the presence of endometriosis. Tiredness, depression and increasing premenstrual symptoms are common associations of endometriosis.

Increased or reduced bleeding or change in the nature of the bleeding may also occur. Spotting between periods is a common symptom.
In some women, infertility may be the only symptom.

The disease often starts in the teenage years and unfortunately, diagnosis is often delayed because of difficulty in distinguishing endometriosis from the more common type of menstrual pain. Hesitation from doctors in performing invasive investigative procedures such as laparoscopy to detect the disease also contributes to delayed diagnosis.

Examination is usually normal, although sometimes swellings can be felt within the vagina in the region of the utero-sacral ligaments.  .Imaging modalities such as ultrasound, CT scanning and Magnetic resonance Imaging (MRI) can be used in diagnosis of endometriosis.
Definitive diagnosis requires surgical (laparoscopy) remove of endometrial tissue and histological analysis in a laboratory.

[i] https://www.healthline.com/health/womens-health/pelvic-pain-in-women#causes