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: 
Users of some SSDs are reporting that `smartctl -A` is giving them a `Wear_Leveling_Count`.

Others, like me, don't seem to have a way to find out. My drive model is reported as `TOSHIBA THNSNK128GVN8`. I have some `Unknown_SSD_Attribute` but they don't seem to have values that seem to be representing my discarded blocks. `smartctl` also says `Not in smartctl database`, I don't know if that may be a factor.
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.