You can't tell strictly from appearances if someone uses PED's. A-Rod does not appear to look like the typical PED user, but he admittedly was / is. Some guys use PED's to help them recover faster from workouts and injuries. Go visit a marine base, you'll see a number of guys that are more muscular and vascular than Tiger who have never touched PED's. Despite the common myth, you can get quite large naturally by working out hard and following a proper diet.
I place Tiger in the same category as I do Derek Jeter (btw, I hate the Yankees). I believe both have so much natural ability and respect for the sport they play that neither would consider taking PED's to gain an advantage.
I think A-roid definitely looks bulked up. But he has admitted to juicing early on, so we've always seen him this way.