Is this the same garter that's only 17 inches or less? If so, that seems a bit small for a garter snake old enough to breed. And the tail should be a lot longer than 1 inch, unless the tip has been broken off in the past.
The only sure way to determine gender is to have the snake probed by a vet or other qualified indiviaual. An unqualified person easily could injure the snake. Then again, I suppose the if it gives birth, then it's safe to say she's a female! 
Checking the taper of the tail often answers the gender question, but not always. Unfortunately, can't tell from your pic. First, locate the cloaca (vent) on the underside, which marks the base of the tail. Then note the taper across this area, preferably by examining from it's side or belly. If the taper is gradual and uniform, it's probably a male. If the rate of taper changes noticeably across the vent area (narrows more abruptly just behind the vent), it's probably a female.
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MichaelB