It depends on hospital policy, and they will usually say they are not 100% sure, but as sure as they can be (just so they don't get in trouble later if they're wrong).
If the hospital policy is to tell you when possible, you will usually need to ask them when you go in for the scan, and I don't see any reason why they wouldn't tell you if OH wasn't there.
We nearly didn't find out as little man had his knees up and ankles crossed over his bits!