1.) Abnormal foliage/leaf growth. A thinning canopy, early leaf drop, early "fall colors", growths on leaves or clumps of leaves indicates a much larger issue
2.) low sucker/water sprouts
3.) sluffing/flaking bark
4.) Fungal/Mushroom growth anywhere on the tree
5.) Splitting or sharp "v-shaped" unions. Usually sporting black/rotting material at the unions
6.) Significant die-back of the canopy may be the result of damaged/unhealthy roots
7.) Vertical seams/bulges running the length of the trunk. This is most likes the result of a Lightning Strike or Freeze Relief Cracks
8.) Decay pockets or holes. Usually housing all sorts of woodland creatures.
9.) Woodpecker Holes, this is a secondary issue to an insect infestation, we can stop this.
10.) Crossing/rubbing limbs, this creates wounds allowing diseases to infect your tree