The ones who have most knowledge about the vedas. Though the coming question can be vague, yet How should one decide his Guru?
Regarding the second question, I assume you are referring to a 'spiritual Guru' which is very different from a 'Vedic scholar'.
Here is a similar question answered by Sri Sri Ravi Shankar:
Q: The country is so full of gurus. I don’t know whom to accept as a Guru. Kindly advice whether I choose a Guru or a Guru chooses me? If I choose then how do I select one? Sri Sri Ravi Shankar: Well, I can’t give you any criteria. You choose. It’s up to you. It’s not something you choose intellectually. Something in your heart guides. You feel at home, you feel cared for, you feel comfortable, right? Otherwise, you can keep asking this question wherever you go. There’s no point, right?
I hope this helps! :)