Hi Paul,
I have breed both the Nematobrycon palmeri and lacortei, there eggs are typical tetra size about half to 1/3 smaller then the common danios and clear(infertile eggs go white), when you see them you will know!!.
(note it's been a long time since i've spawned danios)
You say the females apear full one day then empty the next.
In my experiance it is unusal for this species to drop all there eggs in one mornings spawn, but can happen from time to time when the female is VERY ripe.
More often then not when placed in a spawning setup with Ph 6.5 to neutral and very low GH idealy below 10ppm for max fertility,the fish (trio is best) will start spawning after 48 hours and will breed over 2 to 4 days.
They are egg eater so lots of cover or a spawning trap may help.
Once spawned a pair of N. lacorti in a 250mm sq tank 200mm deep, the bottom of the tank was 100% covered in thick spawning mops, after a week there good numbers of eggs under all the mops, but as I had used water with a GH of 30ppm(tap water) most were infertile.
if your fish are breeding in your com tanks,as it is well planted with lots of cover you should have small numbers of fry turn up, I have seen this happen.
good luck
Stu