The crop should be empty in the morning and full in the evening. It can be various sizes during the day and can be so full that it displaces feathers, looking like a giant bulge and when felt, it can be absolutely full of food, feeling hard.
She may be a bit hungrier coming into lay, so likely nothing to worry about. If you are concerned, do check in the morning before she's had the chance to eat - if it is thick, swollen, mushy or rock hard before eating in the morning, that would indicate a problem.
Sour crop is a yeast infection of the crop and just like a sourdough starter, it will bubble and you'll be able to hear air squishing in the crop when checking. There will also be a doughy sour smell to her breath and you may see signs if the infection at the back of her mouth (will appear white). - the crop may still be emptying, but will likely have slowed down.
With impaction, you'll feel a hard lump or very full crop. This would be a physical blockage, Keeping the crop from emptying.
Also, there's pendulous crop, where the crop sits a bit low and causes the crop to empty slowly.