There is a self-attunement process. It can be done either through intent or by asking your Higher Self for attunements. It is possible to get a manual to familiarize yourself with a system and then self-attune to any reiki system. It tends to be easier if you have experience working with energy because at that point you will be more confident in your ability.

Yes, I think it entirely possible you have attuned yourself, whether you call it reiki or not. The important thing is that it is a source of energy that flows automatically through you as needed rather than depleting your personal supply.