Protospiel and design events
A Protospiel is a convention devoted entirely to playtesting unpublished games. There are no vendors and no finished games on the schedule, just designers testing each other's prototypes from morning to night. The original Protospiel started in Michigan in the early 2000s, and independent Protospiel events now run in dozens of cities across the US and beyond, typically as weekend events with modest badge prices.
The format is simple and powerful. You bring a prototype, put it on the table, and other designers play it and tell you the truth. Then you return the favor. Because everyone in the room designs games, the feedback is a different grade than what friends provide: testers articulate why something is not working, spot mathematical problems, and suggest mechanisms from games you have never heard of. It is normal to test, revise your rules that evening, and retest the next day.
Larger conventions offer a similar experience in miniature. Gen Con, Origins, BGG.CON, and UK Games Expo all have playtest areas (often run by playtesting organizations), and events like Unpub in the mid-Atlantic region focus on unpublished games the same way Protospiel does. Publishers scout these rooms, and many signed games were first noticed at a Protospiel table.
To get the most from an event, arrive with a playable prototype, a thick skin, and a notebook. Do not pitch, playtest. Keep rules explanations short, watch quietly while people play, and write everything down. Plan to test other designers' games at least as much as your own, because that is both the etiquette and where half the learning happens.
Video tutorials
This is why your board game feels like WORK!
How to design a worker placement game with Shem Phillips & Sam Macdonald