Question: I have replaced my laptop's HDD with a 480 GB SSD. 

I left a 20 GB gap at the end of the disk for over-provisioning (the location doesn't matter because it is a SSD). And the main partition has lots of free space.  

I used the trim feature integrated to Windows for the main partition. 
I then **hope** this SSD is correctly trimmed, but I have no proof.

Does someone know a way to find how many trimmed blocks/sectors/percentage/whatever are on a SSD?

As there is not standard for this, I'm afraid this require to communicate with the firmware, which involve different method for different SSD.  

This is not a problem if this require to boot from Linux.

This is a SanDisk SDSSDA-480G 
I already used SanDisk SSD Dashboard, but the values reported are only what Windows reports (this is written in the program).

Answer: 
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.
Your question makes little sense, the [TRIM command](https://en.wikipedia.org/wiki/Trim_(computing)) is at it's most basic your OS informing the microcode in the SSD that a certain block no longer contains data and that it can be wiped/used for wear levelling or whatever.

For monitoring health and wear data either use a tool from the vendor (for instance the [SanDisk SSD Dashboard](http://kb.sandisk.com/app/answers/detail/a_id/15108/~/sandisk-ssd-dashboard-support-information) or monitor the industry standard [SMART data](https://en.wikipedia.org/wiki/S.M.A.R.T.) reported by the drive.