Yeah I had problems in the past with the some of the old Dorman coolant elbow o-rings, on the old plastic ones some of the o-rings were a tad smaller than the OE AC Delco O-rings, and they would cause leaks, I have a thread on another forum about it, I posted pics and what not..
Most of the plastic Dorman elbows sold now should have the O-ring issue corrected, but I'd go with the Dorman aluminum elbows if you ever have to change them out again,

Hi guys -- Well, I discovered that when I replaced the manifold gaskets last winter, I bought the aluminum elbow set, and the o-ring on the large elbow coming out of the water pump was bad. That must have been the source of the leak. Had I used a mirror, I should have been able to see the coolant coming out.

I mistakenly bought the $45 GOPLUS radiator leak kit, only to find it didn't include an adapter for GM or Ford radiators! Sent it back, which cost me $20 for shipping. In looking more closely this time, I find the complete package for adapting to a GM radiator is quite expensive. O'Reilly rents a tester for apparently no cost. What would you recommend? Thanks for everyone'* help. --- Rework

So are you saying you know now that the coolant elbow is leaking, but you are unsure of what to do next?
If so then replace the o-ring on the elbow, then keep an eye on the coolant levels, if levels start dropping like they were before then rent a pressure tester, and try to find the leaks with the system pressurized.