Action beats opinion every time
Anyone will tell you an idea sounds great, because being nice is free. The signals that matter cost the person something: a deposit, a pre-order, time spent on a clunky workaround, or a spreadsheet they built to limp through the problem today. When someone is already paying for a bad solution or duct-taping their own, the pain is real and budget exists. That is worth more than a hundred enthusiastic nods.
Watch for pull, not push
Early on you are usually pushing your idea on people. The signal flips when people start pulling it from you. They ask when they can use it, introduce you to a colleague unprompted, or follow up before you do. Pull means the problem is urgent enough that they are doing your selling for you. Push that never becomes pull, no matter how polished, usually means the pain is not sharp enough to drive a purchase.
Look for a frequent, expensive, specific problem
Strong ideas tend to attack problems that are frequent, costly, and felt by a narrow, reachable group. Frequent means it comes up often enough to stay top of mind. Expensive means time or money is bleeding out today. Specific means you can name the exact person who has it. A painkiller for a sharp recurring problem beats a vitamin for a vague nice-to-have, almost every time.
Be honest about false signals
Some signals look strong but lie. Friends and family saying yes, big vague market numbers, and lukewarm survey checkboxes feel encouraging and predict nothing. Free signups are weak too, because the real test is whether people will pay. Discount any signal that did not cost the person money, time, or social risk. If the only evidence is people being agreeable, you have not validated anything yet.