No. You don’t. (pdf)
Research from the USDA suggests that we are generally wrong—by quite a bit—about what we’re eating and how much. Researchers asked people to estimate how much food they eat in each nutritional category and then to keep records for 14 days of what they actually eat. There was a big gap.
Across age and gender, we underestimate the amount of grains we’re consuming and wildly overestimate the amount of fruit. (We think we’re eating about 2-1/2 times as much fruit as we actually are. And even the fruit we think we’re eating would not be sufficient to meet federal nutrition guidelines.) Generally speaking, we’re pretty accurate about how many veggies we get. We’re consuming about half as much dairy as we think we are. And we’re actually eating between 2/3 and 3/4 of the meat that we report.
The truth is, we eat about the right amount in the meat/protein/beans category. We eat a bit too much in the grains/cereal category. And we don’t get enough of anything else.
Oh, there is one other category that we get enough of: "fats, oils, and sweets." The USDA does not recommend an amount, it only warns us to "use sparingly," a warning that is apparently going unheeded. Women consume about 50 percent more than they say they do and men consume about double what they report. And both genders are reporting that they eat more "fats, oils, and sweets" than they should.
It could be that the first obstacle to good nutrition is simply getting people to accurately assess their eating habits. It’s awfully hard to combat obesity or encourage nine servings of fruits and veggies a day (the new federal guidelines for men) if we don’t even know what we’re eating in the first place.