Official Portal and This Tool
The official place to verify a GST number is the GST portal, gst.gov.in, under Search Taxpayer, which needs a captcha for every search. This tool is not the government portal; it is an independent tool by MarkRTD that reads the same GST registry details for you.
As you type, it checks the GSTIN’s format, state code, PAN part and check digit for free, so a typing mistake shows at once. A valid GSTIN is then looked up for its status, legal and trade name, constitution of business, registration date, jurisdiction and address. Registry lookups are part of the free trial tasks, then a plan.