What evidence in a slice of bread shows the presence gas?

The presence of holes in the bread.

When yeast ferments the sugars in bread dough, it produces carbon dioxide gas. This gas gets trapped in the dough and forms bubbles. When the bread is baked, the bubbles expand and create holes in the bread.

The number and size of the holes in bread can vary depending on a number of factors, including the type of yeast used, the amount of sugar in the dough, and the baking temperature. But in general, more holes indicate more gas production, which is a sign of healthy yeast.