For Reiki to work, belief is irrelevant. The mind does not control Reiki. You can believe in Reiki or not, it will still work.
The mind is pesky and it will ask for proof. It will ask how to know it has experienced Reiki. Your task as a Reiki Practitioner and/or as a recipient, is to quiet the mind and experience the Reiki. This can take the form of showing up — consistently. That means, as a Practitioner, you consistently and regularly practice self-Reiki. As a recipient, you regularly receive Reiki.
Do you need to believe in it? No. Just do it.