D&D 5E Does a monster "know" when a PC is dodging and vice versa?

[The Crimson Binome]

As a DM, if a monster is using the dodge action, do you tell the PCs? Also, when a PC is dodging, do your monsters know this information? Thoughts?

Yes, when someone takes an action, that action is visible to everyone nearby (barring unusual circumstances). Going on the defensive is exactly as obvious as swinging your sword or casting a spell.

If you don't want to give a sign that you're actively on the defensive, then you can ready an action to Dodge (or Disengage and/or move away). Readying an action doesn't give away any vital information, except that you're readying.

 

[Unwise]

For me, if in doubt, people proficient in martial weapons know all this stuff, people without that proficiency don't.