You must read 'The Orpheus Clock' by Simon Goodman.  He is the master of this topic, and he is also around on social media.

Sadly, my widowed German grandmother had everything stolen from her (or forced sale, undoubtedly, at pennies on the dollar), and there was no list of family assets for me to begin to trace.  The only thing I know of was her Bosendorfer concert grand piano, but the factory (Austrian) won't even lift a finger to help me trace it.  I was told that someone contacted my aunt in London in the 50s and offered to SELL it to her, but she couldn't afford to buy it back.

Best of luck!
--
Marc Stevens
Toronto