It's not "surprising" to us that a guy who looks like this or acts like this or consistently has run-ins with the law like this would choose to do drugs at some point in his free time. Doing drugs doesn't make you unique. In fact, over 100 million Americans have tried marijuana, some are just better at hiding it than others. Just as you can make judgments by watching people (don't kid yourself, you can do it) there's also a lot of inferences you can make by watching an athlete's demeanor on the court/field. Some guys are quiet, some guys are rowdy, some guys test authority by constantly screaming at officials. Their behaviors in plain sight cause many sports fans to make up their minds about what types of people they are (fair or not).
We also live in an era where athletes get the celebrity treatment. That means you can make further inferences about their lives. Sometimes their off-field conduct doesn't mesh with their on-field persona. Perfect example? Marvin Harrison. We're not sure if we saw him say five words in his 13-year career. Then, as he neared retirement, we suddenly heard a wave of stories about him in connection to shooting "incidents." Oscar Pistorius is another guy who fits that mold. What little sympathy we had in our hearts felt bad when he didn't medal. Now? Eh...we feel bad for those he beat.
This rundown is an attempt to discern the athletes whom we could've looked at and said "he's probably on drugs," from the guys we hadn't internally accused. If you're wondering where the PED users are, that's another list for another time. Here are the Most Surprising Drug Users in Sports History.
Note: Let's be honest, most athletes (as well as most people) have done recreational drugs at some point. In order to be qualified for this list you had to be a surprising culprit whose drug usage made headlines.