There is a popular Screengrab that was fixed and uploaded to the workshop. Non-admins can't access the menu, but the command still pops up. When you get into a server, type "screengrab" in console. If it pops up, they are using the screengrab. If not, they most likely are not. This does not work if the server has a custom screengrab, but this one is free and is used by most newer servers that don't have money.