How can scientists know for sure that the cell theory is true? What evidence do we have to prove it?

I agree with it. The most widely accepted theory is that the first cells were created by groups of molecules working together to make the basic biomolecules (Carbs, Lipids, Proteins, Nucleic Acids). These biomolecules eventually found each other and supported each other creating cells. Over millions of years these unicellular organisms evolved into multicellular organisms, eventually leading up to humans and other common life forms today.