Sometimes, especially if it's a chain restaurant, you can look up the nutrition information on the web. McDonalds and Taco Bell do for sure. I've never searched for a big chain like Red Lobster, but I'm pretty sure that they do. I am pretty sure that most restaurants add butter to their steaks and so I ask for them not to put butter on it. I do a search on Spark People for the food I ate if I can't find the information otherwise. I choose something that is close to what I ate and put it down.
I try to get things as close as possible to what they really are, so I measure a lot of foods. I try not to eyeball anything. This keeps me honest.
Hope this helped!