The following serve as signposts for possible tactical combinations:

1) Weakened or undefended King.

2) Undefended pieces.

3) Trapped king.

4) Pieces along the same rank, file or diagonal.

5) Open ranks, files or diagonals.

6) Weak back rank

Read The Seeds of Tactical Destruction by Dan Heisman for a great article on finding and taking advantage of tactical opportunities.

Remember that all tactics are based on undefended pieces, a weakened King, and / or a double attack. – Kotov