Place him on the floor in the middle of your living room. Then take two plates and place them at opposites walls. Place a banana on one plate, and a taco on the other.

Then step back and wait for him to crawl to one of the plates, if he goes towards the Taco, he's obviously straight. If however he moves towards the banana, he is a homosexual heathen.

Trust me, though your child has not developed sexually yet, and even though I'm just some random guy sitting at my laptop wrapped up in a blanket waiting for my space heater to warm up, I'm an expert in the field of Inter-gender-baby-sexuality.