Arguably the #1 customer discovery how-to guide is the book Talking To Humans. Here is a jem:
One aspiring entrepreneur wanted to target mothers of young children. She had heard stories about talking to people in a coffee shop, but felt like it was too unfocused. So she tried hanging around school pickup zones, but the moms were too busy and refused to speak to her. Next, she tried the playground, where she figured moms would be bored watching their kids play. This worked reasonably well, but she was only able to get a few minutes of anyone’s time. So instead, she started organizing evening events for moms at a local spa where she bought them pedicures and wine. The time of day worked because the moms could leave the kids at home with their partner. The attendees had a great time and were happy to talk while they were getting their nails done. —
Hat tip to Jim Stanczak!