How to evaluate a category (php question)

Hey there,

Probably an incredible newbie question, but here goes:

I’m customizing my main-index template, and I want to show a particular piece of css when a post belongs to a certain category.

So when a post is of category #5, show the CSS, if not, dont show the CSS.

I know this can be done with an if statement, but as I don’t know enough of php syntax I keep banging my head on something which is probably really simple to do.
Anyone?