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You won't be asked for proof of onward travel when you arrive in Thailand, but you might at check-in when departing India. Some airlines enforce the policy of "flight out within 30 days or a visa" but some don't. Maybe shoot them an email and if they say "no worries" print it out as proof or buy a cheap flight to Malaysia or Cambodia on AirAsia.com. Or wing it and just tell them your plan to exit Thailand. Or maybe they won't even ask.2
I was asked for proof of ongoing travel during my stop over in Hong Kong on the way to Bangkok. I didn't have it as I was getting the train to Cambodia and you can only buy tickets on the day. They made me wait a good hour, telling me there was a good chance I would have to buy an outgoing ticket. Luckily I was given my boarding pass a while later with no explanation and just told to get on the plane. I know a few people who haven't been that lucky and have had to buy a ticket then and there.
