how would i be able to detect if a player is pressing down the CTRL-Button without firing remote events. Just purely from the server ?
Nope. You need to use Remote Events or Remote Functions.
Inputs from the player are handled on the client because they happen on a player’s computer, if you want to detect them from the server there needs to be some communication between the client and the server. This means it is unavoidable to detect inputs from a player, exclusively on the server.