There are many benefits of a domestic cow.
Above picture is of Deoni species from Maharastra and Andhra Pradesh border region. This cow is from
How to buy a domestic cow?
Check its teeth, generally 6 teeth cow is mature and good to have as it is treated as young. Let it roam around and see it from different direction for any disabilities.
Buying from local neighbor or person
If you are buying a cow from a local person or neighbor, see the cow twice or thrice by going there at different times. Touch it, let her feel that you are touching. Try to touch its milking organs. If its not letting you touch or feeling uncomfortable the chances are you are going to have hard time controlling it. See your capabilities and then decide whether it is controllable or not. If not, better look for other cow.
In general, cow doesn't allow to touch its milking organs so its okay but in general if you touching other body parts she should not feel uncomfortable.
Buying from animal fair
If you are buying a cow from animal fair, avoid mediator. If you can't, do not let their decision dominate you. In general mediators would try to dominate your decision by telling different stories, appreciating about cows, giving examples on how this cow is better and why the price is worth this cow.
All check above (Buying from local neighbor or person) description and ensure that everything is fine.
Checking milk and milking organs
Once you have zeroed in a cow, first check for the milk by milking the cow yourself. If you can't then at least try to touch the milking organs. Ensure that all 4 nipples are of almost same size, if not chances are that any one might not have milk. Notice it carefully while milking.
In general you should not trust on what its owner is saying about the cow, so verify it yourself. Even if at the time of buying from animal fair, a cow is giving 4 liter milk, in your house it might end up giving 3 liter only. So decide the price based on that.