How to request a custom customer field to be filled before proceeding to checkout

I have created a custom required field to customer entity.

When customer creates an account, that field is required to be entered, however, there are customers that has not the field with data because they created the account before my modification.

In customer account information form, I have this field also but if the customer never go to account information page, the field will never be filled.

This field is required for the checkout process, so, when a customer is about to start the checkout process, how can I request him to fill this field before actually loading checkout page?

Can you give me a starting point, please?

Thanks
Jaime