They'll run water down every drain and if it drains, they'll state the plumbing is working as intended. 

Inspectors are able to send cameras down the drains to get a better look.

I didn't do the work, but did pay for it, and it was draining with all the issues that were uncapped after the junk in the house was all removed. 

Those were a drain pan issue and a nonworking exterior heat pump that was causing a circuit breaker trip. 

When you ask about plumbing, be sure to separate out the drain lines and the supply lines.

That part if pretty straightforward, especially if you speak with a good lender, but I'd say most important is to make sure you dont drain your cash at closing--you want a good chunk of cash left over to protect yourself from unexpected costs (I bought my first multi when I was 23 and within a month had a furnace go out). 

If it's cast iron drains then I'd check for leaks. 

Closure in place usually means draining tank, cleaning it and filling it with inert material.

Maybe they cut the pipe to drain the system-part of the weatherization process. 

Someone who is experienced in drains and grading to make sure my house does not get flooded.