Thread: how do I know the content of beta carotene in a solution?

Is there any compounds or something that react with beta-carotene in a solution?
beta-carotene is C40H56...

That is the sort of thing that you should be able to easily look up yourself. Especially given that you clearly have internet access.

Are you attempting to determine this theoretically or do you have a sample of beta-carotene that you have to quantitatively determine the concentration? If it's the second, the easiest way to determine this is run the sample through a UV-Vis spectroscope.