These messaging boards allow you to create "accounts" in the form of a name and a password. Many people could create accounts with the name Q and pose as Q, yes, however the real Q is identified through his trip code. The trip code is the letters and numbers you see next to Qs name on the research boards that are assigned based on the password the user created. So even though anyone could name themselves Q, the only real one will be the one with the trip code that is generated from the password Q assigns. Hes changed his tripcode a few times as he moves boards, and he always confirms them with the previous trip code account before moving onto the next one.

Down the rabbit hole we go. Can be a bit of a long story:

You may have heard of the "chans." If you haven't, they are forums in the form of message boards collectively named after the most infamous & prominent one: "the hacker known as 4chan."

Now 4chan itself was derived from a Japanese message board 2ch, but has since branched off. 4chan and its derivates offer certain features including anonymity (unless you self-identify...) in the form of all communications being labeled as from user "Anonymous" and IP address being hashed - hence references to each other as "anons." This is complemented by another feature which allows for the identification/recognition by other anons. Basically a way create a "username" which is validated using a hashed passcode (scrambled output - look up "hashing" or "md5 checksum" for basics) called a "trip code" and secured by an unique password set by the user.

Other anons on the board can see the username and corresponding "trip code" to verify the identity of the poster as being the same user.

https://theqphenomenon.files.wordpress.com/2019/11/img_20191125_132836.jpg?w=1100

Good TLDR: https://theqphenomenon.wordpress.com/2019/11/25/future-q-tripcode-verification/

Just fyi: there are programs that purportedly "crack" the trip codes used:

http://desktopthread.com/Tripcode_Explorer_Diagram_26_09_2009.png

As far as I know, the method the trip codes are hashed is "mostly" secure as long as the admins do not subvert/circumvent such security as was the case when Q announced 4chan was compromised. Migration to 8chan occurred which was verified by Q. When 8chan was shut down, the subsequent move to 8kun was also verified. So basically at every step of the way, Q has verified his "identity" and his authenticity was then proven in what is collectively known as "Q Proofs."

You can read more about this and the "chan wars" if you want more info.