Google has a handy feature built right into its search engine: for some foods, it will give you a side-by-side comparison of nutritional information. It shows a snapshot by default, and selecting the down arrow gives you many more components to compare, such as dietary fiber content and content of different vitamins.
It works best with very simple items — “potato vs sweet potato” gives you the side-by-side comparison, but “russet potato vs sweet potato” gives you regular search results. It also gives information for single items, but beware: “apple” alone gives you results for the computer company (“apple fruit” gives you nutritional information). Its database has some limits, but it’s a great addition to Google’s search results!
If your search isn’t yielding this kind of result, there are lots of complete food databases searchable directly from Google, too — name the food, a quantity, and a nutrient of interest (“ice cream” “calories” and “ounce” for example), and several food databases will appear among the first results — although if your question is simple enough, the answer is probably in the results, possibly in another Google widget.
Give it a try!