A couple of quick checks for cam lean are as follows: Hold the bow in front of you and eyeball down the string lengthwise. If the cam is leaning it should be quite evident in that the string will take a bend as it leaves the cam. Another way is to lay a straight edge along the side of the cam and see if it parallels the string. Most single cams have some lean in them due to the torque created by the cable guard. There isn't a whole lot you can do about it as there is no yoke to twist up to align it. Some bows are quite bad in fact. Of course this pertains to single cams. It also makes the bows hard to tune as it creates sideways string oscillation on the shot, giving right/left tears through paper

There is a good article on Spot Hogg's website about this and the problems it creates. It's listed in their letters section.