These are some signs you need a new roof:
– if there is alot of cracking on the shingles it is a sign of deteriation
– if the shingles are buckling
– if the tips of the shingles are curling, this also causes a concern for winds getting under the shingles and tearing them off the roof.

Yes, you can apply new shingles over existing shingles depending on the condition of the roof.

Buckling is defined as ridges that form along the length of the shingle, with the ridge spacing usually coincidental with deck board joints. These ridges are caused by the shingle being distorted from the movement of the deck. Buckling can occur from any deck type, but is more common with board decks, and less common on plywood decks. Buckling can occur when a new roof is installed, even if the old roof did not show any buckles, when the roof is stripped, the deck mat be exposed to moisture, causing dimensional changes in the supporting lumber. Therefor it is important to re – sheet any areas where there is a potential problem for buckling.