You can validate an idea without building anything. There are a few ways to do it.
You can validate the idea using SEO tools, check if the competitors have traffic and do some detective work to see if they are viable companies.
Check the SEO space, and see how many people are running ads on the main search terms, see how hard is it to rank for the the main keywords. Check if forums exists, and discount coupons for similar products in marketplaces, check for similar products on product hunt and all the other product websites like Appsumo, Capterra, etc.
Look for blogs, Youtube channels, Facebook groups, subreddits. Join the communities and post there, ask questions. Interact one on one with at least 15 or 20 people and ask them about their needs.
Try to interview a dozen or so people and ask them about their pain points.