I don't know how under-the-hood you want to get with this but I'm sure that you already have a good grasp of the basic principles on how a/c works? With this basic knowledge perhaps you can determine 'how much' of the system is working, i.e. you should have a bit that gets warm and a bit that gets cold and make a decision from there? If you're looking for model specific knowledge then apologies for the 'noise'

Edit: Spilling