- still at the idea stage, I began talking to people online on iOS dev forums - iphonedevsdk forum, and reddit.com/r/ios /r/iphone, etc
- I asked my friends who I knew were devs. I live in Poland, and my target audience is mostly US, so their feedback was slightly limited, but still valuable, because I could talk to them in real life
- as soon as possible I went to San Francisco, and went to any meetup I could find through my contacts, on Meetup.com, and Startup Digest
- I try to follow news as closely as possible (e.g. there is iOS dev weekly newsletter), and look for opportunities to engage in the communication. Even without mentioning the name of my project (which is AppCodes.com -- a shameless plug here :D )
- Sometimes I write to bloggers and people who write about my subjects about something I work on, to honestly gather their feedback, and of course to ask if they want to know more about my project. It's always personal.
After a few years of doing various projects, I noticed that it almost always takes around 6 months for the word to go out that I do things, and people start coming back to me by themselves ("are you still doing X?"). With time, finding connections is easier, but not faster - it always takes exactly 6 months :)