HOW TO RECOGNIZE AN INVADER
In terms of bodily appearance THEY are just like us, something that makes it difficult to recognize THEM at first instance, but with a more accurate observation THEY can be uncovered. Here are some clues on how to recognize THEM:
Some of them have a kind of deformity in their little finger, which is straightened.
They have no pulse and no heartbeat.
When they die, they glow red and incinerate, usually leaving no trace. Sometimes just a very little vestige of ash is left. Once analysed, this ash was identified as fibre of cellulose, as stated in the episode "The Peacemaker".
They need to regenerate themselves at regular intervals of time, otherwise they die.
When they are wounded, they don't bleed.
They are not susceptible to external pain, when they are wounded they usually don't feel any pain. The same does not apply to internal organs, since an invader named Blake showed some visible signs of terrible pain in the episode "The Enemy".
They don't show any emotions. It's evident that they control them very well, most of them are cold and emotionless, but some of them proved to have some strong emotions and even fear of death, like Vikki, Anne Gibbs, Blake, Nick Baxter, Claire, Keith, etc..