The short answer is no - there is no way a mod could possibly know. A healer may use more or less mana on a pull depending on a large number of factors, including:
* How good the healer is - are they using the most mana efficient spells
* How good the tank is - are they keeping aggro off the healer, and using appropriate mitigation skills to stop huge damage spikes (shield block, etc)
* How good the gear of the tank is - a healer will use less mana if the tank has lots of HP/Armor etc.
* How much damage the entire group takes - this comes down to how good the tank is, if the group follows marking, and focuses their DPS etc.
* Any many more besides...
I'm afraid its something that you'll just have to learn - however any mana user who is any good should call for a mana break if they need to, or just start drinking - if you see someone drinking, wait for them to finish