How do you tell the difference between a Sn1 and Sn2 reaction in organic chem?

If a molecule is less bulky i.e. more approachable the reaction will most probably proceed via SN2...but if it is more bulky it will preferably take up SN1 pathway...but there are other factors like first of all make sure that a substitution reaction will take place and not elimination (read about E1 and E2 reactions).

THe easiest way to tell that an Sn2 reaction has occurred is that the nucleophile did a backside attack at the reactive carbon, so the stereochemistry has been inverted. Another clue as to which mechanism is working is looking at the solvent in which the reaction is run. For an Sn2 reaction a non polar solvent is best whole for an Sn1 mechanism, a polar solvent usually increases the rate of reaction. This is because the polar solvent will stabilize the carbocation of the mechanism.