I guess the only real way to tell is after they kill you and it shows the death cam if they are stupidly far away where you know they wouldn't really be able to kill you from, of course some can have lucky shots or if you have access to their PB screen shots which would so there being no fog
Death cam is after someone kills you and it shows you where they are, not enabled on all servers.
PB Screenshots are screenshots that punkbuster automatically takes in-game and avaliable to server admins (though not very reliable as there's a lot of reasons they don't always work, graphics drivers being one of the main or hacks to stop it but you can't really tell the difference)
Here's one from Google images showing a no fog hack
There's others such as black sky hack, which makes the sky black meaning it's easier to see things.
And of course aimbot/wallhack which can look like this
you can see all the enemies are shown with health bars