Can a woman tell the moment a man ejaculates inside her vagina?

Not with certainty: Ejaculation can occur without orgasm and orgasm can occur without ejaculation. So a women cannot tell, with certainty, if a man ejaculated. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex.

Sometimes ....: sometimes not. There are fewer nerve fibers in the upper vagina; it is less sensitive. Feeling a man ejaculate depends on the size of the penis, the volume of ejaculate, the amount of lubrication a woman has, etc. One typically appreciates when a man is approaching orgasm but women often don't know exactly when semen has been released. Stay well:)

Ejaculation: The moment if ejaculation cannot be determined with any specificity. Ejaculation can happen without orgasm.