Im currently making a mod where i'm attempting create a class extending ArmorItem that adds a function teleporting any mob that attacks the player a certain distance, however i'm having issues with finding a method that checks if the player has taken damage or been attacked, let alone then checking what attacked them. If anyone could help i would be very thankful.

To check if a player has received damage, you need to use an event. These events are named differently in different versions of the game. To understand which creature attacked the player, you need to use "instanceof" operator. For example:
if(e.getLivingEntity() instanceof EnityMod) { //some code }
The above code is just an example of using an operator in an event. You will have to find the event you need yourself and get the entity that attacked the player