i assume you like the fit on the bike you currently ride. after all, you state that you've ridden 1500 miles in the last five months. so it's a starting point in my book.

as has been mentioned. take a few measurements on it. the middle of the crank spindle to the saddle rail is a good start. and the middle of the seatpost at the saddle rail to the middle of the handlebar where it is attached to the stem, is another. then as been mentioned, compare it to bikes that you are looking to purchase.

in my experience fitting is very personal and varies throughout ones riding career. let experience be your guide. you are unlikely to hit the nail on the head the first time. be patient.

if you are completely lost, i would recommend going to at least three or more LBS's and ask each what size frame they would recommend, then assume the average of them is not too far off the mark. it's probably better than adhering to one approach no matter how compelling it may be.