There probably are distinct signs that the server could detect, and flag up whether or not a user is using xray, but not sure if there's a plugin for it. Feel free to search really.
Edit - I've found an interesting plugin, it wasn't what I was looking for, and it could create more work for Clown... but it would definitely help with players xraying by using texture packs that reveal ores. What I was initially looking for was a plugin that detects, and displays every player's texture pack, in window, or in chat via commands. This way OPs or other players could detect who is using what texture pack, and whether someone is using one which allows them to see through the map. What I found on the other hand was this >>Texture Pack Menu<<.
What this plugin does, as it says in the link, is it allows you to override any player's texture pack, and sets it to default. Okay, this is restrictive, but it allows you to also install other texture packs, and allows the players to choose them off of a menu.
I'm not sure how much more strain this would put on the server, or whether it's compatible, but it's just an idea.
Edit - I've found an interesting plugin, it wasn't what I was looking for, and it could create more work for Clown... but it would definitely help with players xraying by using texture packs that reveal ores. What I was initially looking for was a plugin that detects, and displays every player's texture pack, in window, or in chat via commands. This way OPs or other players could detect who is using what texture pack, and whether someone is using one which allows them to see through the map. What I found on the other hand was this >>Texture Pack Menu<<.
What this plugin does, as it says in the link, is it allows you to override any player's texture pack, and sets it to default. Okay, this is restrictive, but it allows you to also install other texture packs, and allows the players to choose them off of a menu.
I'm not sure how much more strain this would put on the server, or whether it's compatible, but it's just an idea.