I wouldn't look at the fat, I'd look at the calories. It's the calories that will cause you to gain or lose weight. (Fat has more calories in it than the other macronutrients- carbs and protein- but it's not what you should be looking at) And I can heartily recommend the Be Good to Yourself/ Light Choices/ etc ranges, I use them a lot.
I'm looking at the Sainsbury's website, and the "Doritos Sour Cream & Chive Dip" (the fat is right, not sure if that's the one you're talking about) is 244 calories/ 100g, whereas the Be Good to Yourself plain sour cream is 101 calories/ 100g. So the latter is less than half as fattening. You need to be quite careful with things like that, as a serve may be less than you think it is (the more calories per 100g something has, the more careful you need to be). For sauces, less is always better.
To work out how many calories are in what you cook, you need to know the calories of each ingredient and how much of it you use. I do this sort of thing all the time for my calorie counting- the more accurate you want to be (I want to be quite accurate) the more fiddly it is (I weigh all my food- for recipes I weigh the ingredients, write it down, run it through my calorie calculator, then divide by number of serves). Even with my system, which is about as (pardon the expression) anal-retentive as you're going to get, it's not hard, it's just fiddly.
There are also recipes online (such as on BBC Good Food's website) which give you calorie estimates- I don't always find them to be accurate, but they're a good start.