Trees like spikes, are great placements but not many climbers are experienced tree surgeons so it is difficult to be sure of them. First look up, the tree should look alive, with either leaves or in the mist of winter some fresh buds or healthy looking branches. Then look down to see if the roots look healthy and strong. As a rule of thumb the thinnest diameter of trunk that is suitable for using as a anchor is roughly the thickness of you thigh, If it as thick as your waist then it could be used as a single point anchor. Just like metal spikes tying around a tree low down will reduce the leverage on it.