Any way to know how many trimmed blocks are on a SSD?

The data you search for are only inside a private part on the SSD. Currently no standard way to access the internal SSD controler and its data structures. So even if possible it will be device specific.
I suspect the existence of such programs in the hands of manufacturers, but never saw one. That said, I never searched for them.

With a crucial MX300 SSD I found trimmed blocks are read faster than non trimmed ones. I did it on a big trimmed area where I overprovisionned and in an empty partition.
I also tested it against a partition filled with zeros, to check if highly compressed data could lead to the same result: compressed data were a little slower and distinguishable from trimmed areas.
I don't think this will be easy for small areas, thus difficult to know the real count, but easy enought to be sure big areas are ok.
Probably each SSD respond differently. I tested only ONE of them.

In you case this may be ok to verify is the overprovisionned area is trimmed, and if your low-data partition seems trimmed.
For the regular trimming operated by Windows, I think you can be confident.