If a broody hen does get off the eggs to eat and drink, she will usually do it when no-one is watching.
I would start by feeling her crop to see if there is anything in it. If you do find food in her crop, you know she got off and ate. She might do it first thing in the morning, before you are up and looking, or she might do it later in the day.
You could just leave the food and water available, and hope for the best. Put the food and water far enough from the nest that she must get off, because you want her to exercise a little and you want her to poop away from the nest. If you find some food eaten, and a big broody poop, you will know she got up to eat.
If you find that she is not eating, or if you are worried, you can lift her off the nest and sit her by the food and water. She might sit there for a little, but then she will eat and drink and poop and then run back to the nest.
Broody hens do lose weight, but most of that is fat they stored up in advance for exactly that purpose. The egg-making organs also shrink and weigh less. So a certain amount of weight loss is normal and healthy.