I dont know much about Geckos, but I did find this...
"Never have more than one male gecko per cage. Adult males of all types of lizards will fight each other, and sometimes to the death, if they come into contact. A male will vibrate its tail rapidly when it sees another gecko. If the other gecko in turn vibrates its tail in the same fashion then each of them knows that contact has been made with another male and a fight will occur. If, however, a male signals his presence via the tail shaking and the other gecko does not respond in like fashion, then the male knows that the gecko near him is in fact a female. This behavior is one means of determining their sex."
"Sexing your gecko is not very difficult. Male geckos are larger, heavier in the neck region, have a line of small pores on their belly between their hindlegs which are just in front of the anal opening or vent and they exhibit two swellings at their tail base and just past the vent. Females lack the large size, in general, and the pre-anal pores and post-anal swellings are missing. Sex can't be easily seen until your gecko reaches 5-6" in total length. Most of the geckos sold in pet shops are females."