Hi! Let me help you with your sentence. You said, 'if a person is in love, we can see it on a brain scan.'
To make the sentence clear and correct, you can say: 'If a person is in love, we can see it on a brain scan.' This is mostly correct, but starting a sentence with a capital letter is important.
So the correct sentence is:
'If a person is in love, we can see it on a brain scan.'
This means that when someone loves another person, scientists can look at their brain using special pictures, called scans, and notice changes that show feelings of love.