In the TV series "The Invaders", when the aliens assume human form, which part of their body betrays the fact that they are not really human?

Twenty-five years before "The X-Files" went on television and said that aliens were present on earth and up to no good, David Vincent (Roy Thinnes) on the ABC 1967- 1968 TV series "The Invaders" began a lonely quest to save mankind and the world from aliens. In the first few episodes, the show focused on the aliens’ background, actions and steps, and group methods for living on the planet Earth.

To help identify the Invaders from actual human beings, some are again and again identified by what Vincent calls a "deformed fourth finger". Technically, the finger is not deformed. It is immobile; it is always sticking out. Within various early episodes, hints are given that the aliens are correcting this bodily problem so that not all invaders have it. Toward the end of the second (final) season they seem to have eliminated the defect altogether. Another way to identify them is needed.

With the early shows, facts and the premise are to a good extent flushed out on the basic ideas that Vincent and the 'Invaders' are involved in a very plausible winner take all world wide conflict. The ground rules for how they will react with each other are clearly set.