as was stated above blood test , look for labs that do freemartin testing , as they carry a Y chromosome , other option is around a year have them palped for breeding soundness , at her age she should be having heats , as my 3 1/2 month old high percentage heifer is already having signs of heats , I did have 1 checked at about 400lbs that I bought , as I did not see any heats in the 2 months I had her , and turned out she was 2 months bred , so vet aborted her , AIed her with sexed Jersey at 650 to 700lbs and she is due with her 1st calf in November
Suzanne