The Steam ID checker is a quick and easy way to find out who is on your server and which team they are registered to. Simply use the drop down menu to select the game type you are querying (remember this is a STEAM ID checker.... it isn't for all gametypes) call a status command in your console and starting from the line below - paste the rest of your console into the form and press submit. You will now have a list of all the players on your server.
# name userid uniqueid frag time ping loss adr
Please bear in mind that sometimes where a player is registered to a CS, a CZ and a CSS team then one of the other teams they are registered to may be shown instead of the one you are expecting to see - so if it isn't the right team make sure you at least click on their name (which will appear as a link) and check what other teams they are in before challenging them on the server about "being in the wrong team".