They would know if they see anything like traces of water or anything else that would be nessecary for a living organism to have. They look for these with technology like the hubble telescope as well as the drone on Mars to gather as much information as possible.
Trilateration Measures Distance, Not Angles
Using a simple two-dimensional example, let's imagine we have three GPS satellites each with a known position in space. ... This means that the distance could be anywhere on that circle. But this time, we have two known distances from two satellites.
The common characteristic of those two organisms is hard spherical shells (exoskeleton).
Foraminiferans are single cell marine eukaryotes divided into granular endoplasm and transparent ectoplasm. Foraminiferans are enveloped with tests, hard shells, usually composed of calcium carbonate (sometimes from organic compounds or silica).
Coccolithophore is a unicellular, eukaryotic alga with special calcium carbonate plates (or scales) of uncertain function (coccoliths). Each unicellular alga is enclosed in its own collection of coccoliths, the which make up its exoskeleton- coccosphere.
