Hi ameeyal,
You are right, if there is a plentiful supply of grass, they can cope with just that, as they do in the wild. Where grass is in short supply, wheat is usually fed in a little water.
At this time of the year, it's usually hard to keep sufficient fresh grass for them with sufficient nutrient quality.
If the ground has been well used, there is a good chance they are carrying a high worm load and I would consider worming them - flubenvet is the most effective in this sort of situation, it will kill hairworms, gapeworms and all of the main stomach worms. If the ground is heavily infested, repeat treatment is a good idea = this means the worm eggs that they pick up (that re-infect them) don't reach maturity and lay more eggs. 3 weeks is a good time to leave after the first treatment but it does vary between worms and in an ideal world, you should send off a sample and get it tested so you can choose the optimum repeat treatment point.
To answer your question... which (sorry) I have not done yet! You can either examine them and look at the breast bone as you are doing, or, alternatively weigh a goose to check they are putting on weight. A hand held scale with a loop of rope around the gooses feet can give you the weight quickly. The goose will cope being upside down for a short time while you weigh him.
To catch them is another problem - I have made a small pen in the past, like you would for sheep and herded the geese into it.