Unfortunately there's not a really easy method, if you want to be 100% sure. Probing is the only...almost 100% way, and hemipene eversion is only reliable if it's a male....cause they'll be there, but if you don't see anything it doesn't guarantee it's a female and you can only do it with young snakes. Pretty much all garters have long thin tails (makes sexing them harder then other colubrids). What you want to try and look for is to see how the tail comes off the body. If after the cloaca (the opening at the base of the tail) the tail really constricts or doesn't smoothly continue it's probably a female....if the tail stays thick and doesn't slant sharply after the cloaca, it's probably a male. The secondary sexual characteristics that you mentioned do make it sound like a male, but again, it's no guarantee. Good luck...oh and good looking snake, whatever sex it is.
