How do you validate a startup idea before investing time and money into building it?

hemortring75

I have a concept that seems promising, but I don't want to spend months developing it only to find out there's no real demand. I'm interested in practical ways to test market interest, identify the target audience, evaluate competitors, and collect meaningful feedback before making a major investment. For those who have launched a company or digital product, what validation methods worked best, and which early indicators helped you decide whether to move forward or change direction?