Probably one of the better ways to detect a leak in the system is pressurize the system with nitrogen and use a bottle of soapy water and spray all hose connections, connections at all of the components in the system and any area that looks oily. A sniffer is probably the only way to try and detect a leak in the evaporator and you will have to use that at the drain for it.
As far as adding oil, there is no way to determine how much oil is left in the system if you have a leak. Most of the systems that I've worked on have a total oil capacity of about 8 oz and that is spread out between all of the major components. I wouldn't add more than an ounce or two if you think it's low.
STP have confirmed the top-up I used didn't contain any dye which is a shame.
Debating whether or not to purchase Liquimoly 20736 which is a compatible oil that contains a dye or just to purchase a specific dye kit to put in the system.