When will they be able to tell............?

They will start checking for the height of your fundus [measuring your stomach], as well as checking for dialating and effacing. As far as breech babies go. They can tell now, but babies flip all the time. So just because your baby is head first now, doesn't mean at 40 weeks he won't be breech. This is usually something they check on towards time to go into labor, or for some women, when you actually go into labor.