How do I know if my son's football coach is being too hard on him? I want him to be pushed but not too hard.

If you are involved in your son's football then you will probably have the opportunity to observe the relationship between your son and his coach. If you have a sense that the coach is being too hard then ask your son about his experience of football and make enquiries about his experiences. Communication is key here. Don't make assumptions and do listen. Make sure you you don't ask questions in a way that feels too invasive or presumptive. You are making enquiries not assumptions. 

Ask questions like "Tell me about your experience of football?" "Are you enjoying the time you spend at football?" See what your son says both verbally and non-verbally. If he looks away and doesn't answer then ask a more direct question like, "Do you wish to carry on with your football?" 

I believe if you listen carefully and observe you will know what your son is experiencing. If you find he is generally uncommunicative then look to improve your relationship and your communication.