Normally, before we decide to begin relaying, partner has shown something about his HCP strength, and maybe something about his distribution. I will call this stage zero of the relay bidding, because it can happen before relays begin. Relayer probably knows when there is game or possible slam, before relaying.
Responder shows number of controls .
Of course, relayer can stop short at any time, depending on the situation. The details may vary, sometimes quite a bit. The details may depend upon the partnership preferences, or upon the situation. For example, in the Ultimate Club, when responding to a 1C opening bid, responder shows number of controls right away. Also, Blackwood or other conventions may fit in somewhere.