State three factors required for there to be a chance of life existing somewhere other than earth

Water which is an a liquid state at 'regular' temperatures on the planet
Some kind of energy source (this will be different depending on what type of organism it is, e.g. plants may be different to bacteria etc) 
Organic compounds (these always contain at least some carbon) as these are needed as reactants in biological processes, the processes of such reactions can act as 'building blocks' from which life can begin.
I don't know what level of detail you need or would like for your work (or more importantly how interested you are in the subject) but the Miller-Urey experiment is all about how life began so covers all these 3 stages and may help you understand the work in more detail

They are like other investigations? say there was a murder u have to do blood test and finger prints which run through a lab.