Oil Leak

Ok, my engine has been leaking oil since I built it, its not leaking really bad so i just pushed off the problem a little bit. It seems to be leaking down by the oil filter "area". At first I thought it was just the oil filter not sealing correctly but ive since changed oil with new filter and still leaking. The thing is I cant tell exactly where its coming from. Is there an oil gallery plug down there in that area i might of forgot or didnt get tight enough? what about where the oil filter inlet/outlet is connected to the block, is there a gasket that was supposed to be there? I dont remember putting one in. Is there any engine oil dye that anyone has used/trusts. Ive heard some of the dyes are not good on your engine......might have just been a myth though. Any insight?