Honestly, there is no real way of knowing until you collect. Again, you want to keep away from the areas that are 'no brainers', like power plant discharge areas, heavy boat traffic etc. Other than that, you need to test the water. Look for high phosphates, high nitrates etc.
If you are worried about petroleum based pollutants, more often than not, you can see it in the water. It would show up as a 'slick' of sorts on the top of the water.
Dave
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Experience in aquaria, 35 years. Experience in marine, 20+ years. Experience in Reef Keeping, 6 years. Always a newbie! | 
