The best way is to search the postcard sellers sites.
Several factors come into play, besides condition.
Unused or used? PC collectors want pristine. Stamp collectors look for those with first day of issue stamps and commemorative postmarks. *Some of these are quite valuable.
Illustration- images by a recognized political artist? Or just static pictures of Lenin? The latter are too dull and too common to be of much interest.

* I collect Japanese stamps. A postcard with the 1900 stamp commemorating the wedding of Crown Prince Yoshihito is worth 12,000 yen or more if it has a first day cancellation, and about 300 yen if dated later.