How could we know that either a reaction goes through Sn1 or Sn2 mechanism?

only occur for alkyl halides  the nucleophile can attack from either face of the flat carbocation.

To determine wither a reaction goes though Sn1 or Sn2 mechanism, we may look for solvent type: (A) If solvent is polar protic, then either reaction will go through SN1 (B) if solvent is polar aprotic, then reaction will go either through SN2