It isn't obvious until you've worked with a few and have developed a touchy-feely sixth sense.
An aluminium nipple that's frozen onto a spoke just won't budge until the nipple either rounds off or breaks in half. Signs of aluminium corrosion by way of white salts may also be visible, depending on whether the nipple was anodized or not. This is easy to ID. Only aluminium nipples can be made red, pink, purple and green. So, if you see a nipple in these colours you know without a doubt. Both aluminium and brass nipples can be made black though.
A brass nipple with Loctite on behaves in one of two ways. If the threadlock/Loctite was of the hard set type, the nipple with resist at first and then pop loose and come of as if there was no threadlock on it. If it has soft threadlock on it, it will be very tardy with its movements throughout the length of the thread.
If it is heavily soiled and impacted with fine dust particles inside the thread, typical of real-world mountain bikes, then the nipple will squeak, groan and complain as it unscrews.