doubt its your evap coil but yes that is the way you find that leak, look in the drain tube for the dye because it will show up.
Another spot to check is the low and high side service ports, these can leak. I think you can replace these but don't take my word. Otherwise just replace the whole line set as they are cheap.
One more spot to look is around the compressor housing, mine was leaking there. This one can be tricky to find because as the compressor gets hot it actually hides the dye but under careful inspection and black light you will see it.
One more thing you can do is vac your system down and fill it with nitrogen to 100 psi and soap all the joints or listen for a hiss. This is what I typically do.
Final option is to buy a refrigerant sniffer and these will almost always find your leak or get you in the area.
good luck, worst case just replace the whole dang thing (condensor, line set, drier and compressor) all the parts is only like $500 and is all labor to get in and out.
