Possible Anti-freeze leak?
 

So last month i started throwing codes and before i found out it was my o2 sensor i checked all my fluids. Anti-freeze was low so i topped it off. Within the last month i've fixed the o2 sensor but now the antifreeze is at the bottom of the reservoir again. Whats the best way to go about diagnosing this?
I've heard a lot about the wonders of white shepard but what is it and in what applications does it/does it not work?
Sooner or later i'll get this truck to a point where it doesnt need any attention (this is what i tell my self so i can sleep at night).

You will need to pressure test the cooling system and look for the leak.
If it is internal your best chance of finding it is by pulling the spark plugs out and looking at the condition of the plugs.