Earnings
The grains you have stored in your house reflect on your earnings. If you earn a lot of money, you have more grains.
Imagine looking at the grains in the houses of people who make vastly different earnings. The poor artist has only a few sacks of grain. The rich samurai has an extra house just for holding his grain. That's how you know he has much higher earnings than the artist.
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Another way to check someone's earnings is to see if they have a car (か). Would someone who doesn't make a lot of money have a car? No!
Picture your own earnings. Can you afford a lot of grains and a car?