The most likely problems are visible around the drain and overflow, and the overall finish of the interior. Porcelain enamel tends to lose some gloss, especially if not treated gently during its life.
The lifespan of a steel tub would probably be listed at about 15 to 25 years. Some get "killed" in less time. My steel tub was installed in 1986. The finish shows its age but I do not have any rust "cancer" at the drain or overflow. No immediate plans for a bathroom remodel. Good for a few more years.
If you are going to all the trouble of doing the tile, I would at least take a close second look at any decision to NOT replace the tub.