What do you mean?
* How we can tell what MID code a disc has: cdspeed2000.com, dvdidentifier.cdfreaks.com, et al.
* How we know that MID codes exist: publicly-available specificiations (Mt. Fuji/INF8090, ECMA-279,337,...,etc. specs, et al.) ... MIDs are no secret; drive manufacturers sometimes even include codes in their published media support lists, and Philips Licensing has posted a list of "approved" + media codes