Molecular Cloning- Blunt end restriction endonucleases
To know if the ends are compatible see compatible cohesive ends (isoschizomers) from NEB website. 

If you are unsure then do a blunt end ligation. For blunting a sticky end (From the comments):

Use Klenow or Pfu polymerase (or any proofreading/high fidelity polymerase).

You can use Taq if you intend to to a TA cloning.