Watch that Lean
A leaning tree is not necessarily a cause for concern. If a tree has been leaning for most or all of its life, it has likely put on wood in the right places to adapt to the lean. Trees may lean in response to surrounding trees, available light, simple genetics or other reasons.
If you notice things like a change in lean, soil heaving around the base of the tree or cracks in the stem the tree should be further evaluated. If you are concerned about any leaning tree, have an arborist perform an inspection.