How can you tell?
point your url bar to about:telemetry
It shows you all the data that has been gathered. (Though IIRC it might still show stuff even when you've disabled telemetry -- in that case the data is being aggregated locally but not sent.)
Go to https://telemetry.mozilla.org
To look at the data on the server side. There are more sophisticated ways of querying it, but obviously not everybody can just be handed access to run arbitrary analysis code.
Probe dictionaries:
https://searchfox.org/mozilla-central/source/toolkit/compone...
https://searchfox.org/mozilla-central/source/toolkit/compone...
https://searchfox.org/mozilla-central/source/toolkit/compone...
How you probably think it works in Firefox and how it actually works in Firefox are two different things, by the way:
https://feeding.cloud.geek.nz/posts/how-safe-browsing-works-...