This is a second floor plastic type one piece shower. It is in a 4 year old home that has been empty for 1yr or more. I believe the leak is around the initial drainassembly somewhere. After removing the screen, I can see brass nut assembly with a rubber gasket going into PVC (I can see water standing in the trap). What would be the best way to identify the specific problem? And how do I fix it?

Before you can begin to repair the leak you must first locate it. Some tests should be done before you open up the ceiling. Let's start with the drain. Before you go to bed remove the strainer and put some wet rags in a baggie and stuff them down the drain to seal it off. Now run about a inch or so of water in the shower floor and let it set over night. Make a pencil mark at the water level. Next morning check the mark and your ceiling. Water level down? Ceiling wet? No? Then let the water out and check the ceiling again. Still no wet on the ceiling? If the ceiling stays the same then it's a pretty safe bet the shower drain,trap and drain line's OK. Do these test and let m,e know how they come out. One piece shower. Does that mean the shower base and its enclosure are all one piece with no joints? What type of door does it have? Let me know, Tom

Will do and I will get back with you. I did plug off the drain with a rag (not a great seal)and the leak was evident. Certainly did not take over night. It is a one piece as you described. The doors are sliding glass in tracks top and bottom. When I was doing the "testing" (rag in pipe) no water got on the floor outside and adrip through the ceiling was evident rather quickly. A little water was probably getting into the drain pipe also.

when I was doing the "testing" (rag in pipe) no water got on the floor outside and adrip through the ceiling was evident rather quickly.
This leads me to believe that the seal under the drain lip's ruptured,(see image). This happens when the shower base hasn't been bedded and will, flex or give a bit as you step into it. Over time this will cause the seal under the lip to leak. The solution would be to loosen the nut, clean off the old putty seal and run a small rope of plumbers putty or silicon jel under the lip and tighten it back down. If you have to cut out a section of ceiling to get at the nut then now would be a mighty good time to bed the shower base with cement or mortar. Simply puddle cement or mortar under the shower floor, especially around the drain,and let it set up and cure before stepping in the shower. Good luck, Tom

That is about what I expected. Minus the bedding. The mortor goes on the bottom of the subfloor beneath the shower? The basement has really nice textured ceilings, and it would be very nice if there was a fix without cutting from the bottom.

The basement has really nice textured ceilings, and it would be very nice if there was a fix without cutting from the bottom.
You can make a square cut, save the piece and feather it back in after you loosen the nut, rermove and clean up the old drain seal and make a small rope of plumbers putty or a ring of Silicon Jel, tighten the nut and clean up the material that's been squeezed out.
If you let the leak go on you'll have to replace the ceiling anyhow. Better to do it now while the ceiling can still be saved. Good luck, Tom