Yes it is easily detectable. It is sprayed in and I believe baked on. This is not the best picture but you get the idea.
Some folks have had luck burning them out. I did not and gave up on this particular barrel. You need a roaring hot fire to get rid of the liner then you need to wire brush the remains to bare metal. I would have gladly paid money for a clean one. As it turns out, I was able to get a clean one for free and the drum in the picture is still in my backyard.