When casting counter spell, do I know if the spell is an action or bonus action?

related to What do I know, when deciding whether to cast Counterspell? except that my particular question isn’t included in his question.

I’ve been thinking of ways to counter counterspell, normally you can trick your opponent by casting low level spells, the issue with this is that even though you burn their spell slots you are potentially losing actions. Burning enemy spell slots isn’t usually very useful if the creatures you face tend to have all their resources at the beginning of a battle. Hence I figured the best way for me to counter counterspell is casting low level bonus action spells.

But I had a thought, can a monster hold off on their reaction until your spellcasting has taken some time and they realize it isn’t just a bonus action? This goes both ways. Should I be able to wait to use my counterspell until I know it isn’t just a simple quickly cast spell?