1971 Ford F-250

I am listing a truck for sale for my mother. She says it has a rebuilt motor with around 1,000 miles on it. The motor is dirty and my mom says it's because
there is a hose missing and it's blowing dust on the motor when it's running. I have done a few repairs on my car with the help of youtube, but I have no idea how to tell if it's actually a rebuilt motor. The motor has oil on it too. Would someone rebuild a motor and not clean it before putting it back in? Is there anyway to tell if it's been rebuilt? I do know it's not the original motor, but beyond that I'm going by what my elderly parents are telling me. I'm posting pictures but I know I'll need more detail. *
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