How do I check if a Stevia buyback company is genuine before signing?

Good that you are checking before signing. Do these steps. First, separate the planting material seller from the buyer; if the same company sells you costly plantlets and promises buyback, treat it as a red flag. Second, ask for the actual processing buyer's name and address and independently call or visit them to confirm they are buying and at what price, do not rely on the agent's word. Third, talk to existing farmers who have already sold to this company and got paid, not just farmers who have only planted. Fourth, get everything in writing on a stamped agreement: dry leaf price per kg, quality and moisture standard, quantity to be lifted, payment timeline and who bears rejection. Fifth, check the price they quote against neutral market sources. If they pressure you with urgency, guaranteed riches or refuse written terms, walk away. Start small even if everything checks out.