My receiver is 100 Watts per channel so I assume even at full volume I can not damage the speaker but does it mean that optimal usage is with only 40 Watts input?
That spec means nothing. At best it means you need at least a forty watt amplifier to go with these speakers. As far as "minimum" wattage goes for a speaker, that's somewhat on the high side. I assume these speakers are not high sensitivity designs.
How would you know when you were using exactly forty watts?