Trichomes play no part in sexual identification of cannabis plants and have no link to determining the sex of a plant.
Males however produce trichomes in only limited quantities, maybe 5-10% of that of a female, but not always.
There is no way to say for sure that because a plant has trichomes early, it must be female....but it would be quite rare for a male to have them this early, but not impossible.
The only way to tell the sex of a plant with 100% certainty, is to wait until the plant produces pre-flowers.