The trailers are fairly small for that age (in my experience) and the dorsal/anal fins are showing signs of black (but it also might be lighting, stressed, etc.) so I would go with female for this one. A sure way to tell is to vent the fish:
A male's vent should look like this: (head)(body) O> (tail) with the O being the anus and the > being the male's genital papilla
A female's vent should look like this: (head)(body) O) (tail) with the O being the anus and the ) being the female's genital papilla
If you are inexperienced at venting just take a picture and post it in this thread.