If you suspect player X to be cheating, the spectate them and record a demo (record [demoname] in console), the re-watch it with something like r_drawothermodels 2 on.
Chances are though you will hardly be able to tell when someone's cheating.
I just remember when i was admin back in Medal of Honor i could force screenshots on people hacking. They were sent to my folder and i caught a lot of people with chams.
Well the main difference between Counter-Strike: Source and Medal of Honor is that Counter-Strike: Source is not Medal of Honor.
CSS is technically more complex in the way it handles clientside textures. The best thing to do is sign up to use steambans if you're that worried, as they have (mostly) competent admins checking for cheaters and if they catch any, they're automatically banned from your server.