What I have done: post a brochure site, promising more to come, and offering a free whitepaper/guidebook in exchange for an email subscription. Use free $100 Adwords coupon to send traffic there. Count email signups. Send out a newsletter to them, try to gauge engagement.
Other tactics I know of:
- Offer pre-sales
- Talk to business owners casually
- Go to trade shows and chat
- Offer surveys
- Ask mechanical turk for feedback
- Lurk in forums to see what people are discussing
- Surf Amazon reviews to gauge how people feel about X