there isn't a single rule for this. because, well, if something has 15 g of fat per serving, what it is dictates how good it is for you. for example, nuts are supposed to have a lot of fat and a serving of those with 15g fat is what they're supposed to be. but if you're talking potatoes, those don't have very much fat normally so 15 g in a serving means there is a lot of added fat in there that you probably don't need.
one of the best things that you can do is to mouse over the articles and videos tab and click on nutrition. there are a ton of articles about various nutrients that you can educate yourself with. keep in mind that this is a complex topic and you're not going to make it through all the info in an hour or two or even a week or two. so don't beat yourself up if it takes some time to learn about what you need to know.
-google first. ask questions later.