Hard to check if the domain is valid. In the best case they can tell if the format of the string you’re giving is a valid domain format, and if the port 80 or 443 of this domain doesn’t return a 404.
The best thing to do to check if the website is valid would be to crawl it’s sources content to verify if the JSECoin code has been integrated to the page, it would work perfectly but would be an extra step. I’m supporting this idea though.
This is in development actually. We have a site crawler to check that JSEcoin is on the page. It’s currently being used more for fraud purposes but we could integrate it for this as well. A “check i’ve installed the script correctly” button would be useful.