Answer:
Hey
Good question
How can we know if I planet was once inhabited by life? There diferent tequnecs for this. The first would be searching in the soil for fossilized microbes. in June of 19, 1976 Viking 1 was launched on its way to mars to began the search of life els wear. What this little lab did was drill a hole in the marsion terrain and take a sample of its soil to search for microbes or microbe fosles. Sadly it did not find anything. Other technics are to search for ruins on the planet or exoplanet. I do not have much to say about this method because it has never been used.
Hope it helped (i wrote all that).
Spiky Bob your answerer
Below is a picture of Carl Sagan next to the Viking Voyager 1. Obveosly this is just a modal. you could not breathe on mars.
