How do I check milk yield before buying?

Milk the cow in your presence and watch the full let-down. Strip each quarter separately into a measured container, then total the milk. Do it after the seller has not fed extra water or hidden the calf. Also check the same cow again at the next milking if possible. If the seller refuses, walk away. For a cow above fifty thousand rupees, take a Pashumitra or vet with you before paying.