I had just reached an international level, but I was never at the olympic level. But a whole group I trained with for a time all became professionals, and some of them competed in the olympics. I knew them then before they got there, and I knew their level and their attitudes, and I saw nothing in their professional performances indicating that they had at any point moved into doping. How sure can one be? Never totally sure. But trying to be "sure" about the other possibility (that they were doping) will need more than "yeah, I'm sure they do".

EditAdd: I did know people who were fascinated about EPO-like medication, and asthma medicine when used without actually having asthma. That was when they were quite young, and that's also a long time ago. I never ran into any of them actually using anything, and they were in the minority - I met just two, and sadly one of them died in a traffic accident before he got to a professional level.