how can you tell if you are getting taxation after you conquer a colonie?

go to my castle/colony tab and go to management.it should come up somewhere saying resources gained from taxes

You can also check how much tax your conquer is paying you by clicking the 'Orders' tab, under 'Conquers'. If they're rebelling, it'll also be highlighted in red. Even though a conquer that rebels get no income per tick while rebelling but you will still get the tax till they are free.