When the label says "Calories from fat 15" then that's what you use to determine the percentage. Take the calories from fat, and divide by total calories.
For this example: 15 / 70 * 100 = 21%
That means the fat intake for this is 21% of the total calories, which is a safe amount. As Heather posts on How to Eat for IBS "Keep your fat intake to 20% - 25% of your total calories".
Hope that helps clear things up, as for times when that "Calories from fat" amount is not shown, I'm not sure what the calculation is.
If I'm totally wrong on this please someone tell me! I've been following this.. Thanks, Kat