1) How to find out if the selected part is loaded as resolved or lightweight?
2) If loaded as lightweight, how to resolve only the selected part (not all of the lightweight parts)?
When you load an assembly, each component will either be fully resolved or lightweight (1 - noted with a feather icon beside it) - To fully resolve an assembly, 2 - RMB at the main assembly, there will be an option to set lightweight parts to resolved.
SW Professional 2015 SP 5.0
SW Electrical 2D/3D SP 5.0
64 bit
Intel Xeon X(R) CPU 3600 @ 2.80 GHz
8.0 GB of RAM
NVIDIA Quadro 2000
If your assembly has some parts loaded lightweight and some loaded resolved and you want them all resolved then you can do that easily without having to check the lightweight/resolved status of each part. Just RMB on the top assembly icon and select "Set Lightweight to Resolved", similar to what Nella95 said.
I don't mess with API so I know this isn't exactly what you are looking for, but you might find the right code by doing this while you record a macro. Then you might see the code you need inside the macro.