How can I tell if the food inside a pressure cooker is done cooking?

It is also like Schrodinger's Cat in the sense that hopefully whatever is inside is not still alive. But seriously: the main thing is time. You start with the recommendations from your manufacturer (since all cookers vary somewhat in terms of pressure and therefore temperature) and then keep good notes as to whether you prefer slightly more or less time. For example, I've learned that with my cooker, if I'm making pinto beans without soaking, for whole beans I like about 35 minutes, but if I'm going to puree them, 40 is better.

Smell.

Every time the whistle of a pressure cooker blows, you get a mild aroma of the food along with the escaping steam. The aroma is usually a good indicator of how good the food has been cooked.

Besides smell, the only other thing is time.

With respect to this kind of pressure cooker:

My suggestion would be to count the number of whistles rather than looking at the clock. These cookers produce loud whistles which you can't miss even in sleep.

For the tender lentils like:

• Masoor Dal (Red lentils)

• Toor Dal (Yellow Pigeon Pea)

  1. You need to soak them for half an hour.
  2. You need to add a little less than half table spoon salt in the cooker.

W.R.T above conditions, it takes 3 whistles to get them done.
At first the gas flame has to be on high. After the first whistle, it is necessary to put the flame on simmer.

For the hard lentils like:

• Kidney beans
• Chick Peas

• Bengal gram (Chana Dal)

  1. You need to soak them for 12 hours.
  2. You need to add a little less than half table spoon salt in the cooker.

W.R.T above conditions, it takes 7 whistles to get them done.
At first the gas flame has to be on high. After the first whistle, it is necessary to put the flame on simmer.

For white rice (without soaking) it takes 2 whistles to get it done.

For the rice, the flame is to be kept on high till 2 whistles.

After the specified number of whistles you are supposed to turn the gas off and keep the cooker as it is on the gas itself for 15 minutes.
The inherent heat from the gas stove and the steam formed in cooker will do their job in next 15 minutes.

My suggestions above are based on my personal experiences with this cooker.

In some cases, you can slosh the pressure cooker around to get a sense of the ratio of liquid to solids inside. This gives rough estimates of progress for things like rice, beans, etc, but takes some practice.