There is a passive way of doing this - by affecting Leon with poisoning, slowing or some other effect that shows some thumbnail above character.
There is also a neat trick usable at AL Station 404. Between the front turret and the top turret behind there is an area with yellow floor that lights up when you step on it. It actually saved me once or twice because I could see invisible Leon approaching me by looking at the floor lighting.
Upd1: another trick, barely usable though. Invisible characters still collect solar 8p. So, if there is solar lying around, you might notice invisible's presence.