What i should be looking for?

[Annies]

For the broccoli to produce seeds for next summer garden, how do i know what i should be looking for?

[ZandraJoi]

I personally have never saved broccoli seeds but it's doable. Let the broccoli "bolt" which means go to flower.
This article would be beneficial to what you are asking for:
https://bloominthyme.com/save-broccoli-seeds/