How do you tell when SN1,E1, SN2, or E2 reactions is occuring (Oganic Chemistry?

How do you tell when SN1,E1, SN2, or E2 reactions is occuring. what do you basically look over. Could some one please explain it in a simple manner and give some good examples. also what is solvylosis? how do you know which reaction will occur? and explain the difference between all four of these reactions.
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