How can i tell what day duck eggs are on?

[Pierre duck drake]

Two of my ducks went broody, and I don't know what day the eggs are on! I'll post candle pics later or tomorrow.

 

[everyday_farmlife]

It's up to you but personally I'd let them just be broody
You don't need to guesstimate the age of the eggs if you leave them with mother duck, she'll take care of the whole lockdown misting humidity ect without causing you loss of sleep.
If you want to do the whole hand raising thing you can take most of the ducklings once they hatch. (Yes I have friends who do this with chicks as they have bad experiences with them getting squashed going missing ect) I personally let nature take its course, they were created to reproduce (true not every animal makes a good parent, but, I dare say most do).