Great question, and honestly something everyone should check before handing over any money. First, look for their GST number on the website or invoice — any registered travel company in India must have one.
Then search for them on Google Maps and check the reviews, not just the star rating but the actual written feedback. If a company has been around for a few years with consistent reviews, that's a good sign. Also ask them for references or past customer contacts — genuine operators won't hesitate.
Always avoid paying 100% upfront; a token amount is standard but paying everything before seeing any confirmation is a red flag. Make sure you get a proper receipt and booking confirmation on their letterhead.
When you are booking a tempo traveller in Delhi, also verify their office address physically if possible, especially for longer trips.