On my last two strains started from seed the plants have showed sex during veg'. The first was a feminised grapefruit, showed after 8 weeks veg'. The second, and most recent was a la confidential x mystery haze, they showed after 6 weeks veg'. BTW, these were all grown in 2 gallon buckets, this matters... pot size matters in relation to how long you veg' the plant. After only 48 hours of flower these girls were covered in pistils and were starting to gain flower heads.
Give your plants those extra few weeks veg (growing from seed) and you will be paid dividends when you flower.