How can one determine if they have an enlarged prostate?

Clinical or imaging.: A digital rectal exam by an experienced urologist is one way, though there is significant interobserver variability. The best way is by transrectal ultrasound (trus), where measurements can be made & volume calculated. This test is usually unnecessary as urinary obstruction is a clinical diagnosis, not completely related to size, and prostate medications are given to see if there is a response.

Different ways: Rectal examination can accomplish this, while transrectal sonographic measurement is more accurate (but more invasive). However, i suspect you may be equating lower urinary tract symptoms (luts) with "enlarged prostate" which is common but often incorrect. There is little correlation between luts and prostate size. You can find an aua symptom score (aka "ipss") questionnaire online score yourself.