You should be able to do one of two things. Either go to a good parts store and see what the seal looks like, or look up underneath and see if there's a flange from the seal sticking out. If there is a flange, it's external. If not, the timing cover is gonna have to come off, so be sure to pick up a timing cover gasket set (Fel-Pro makes an excellent kit).
p.s. be sure to inspect the harmonic balancer for a groove in it from the seal. I'm willing to bet you have one if it's the original. In this case either replace it entirely or get a repair sleeve.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch