I wouldn't say they're synonyms in English. "to fit" means (here) that the trousers are the correct size, whereas "to suit" needs a direct object and means that one thing goes well with another "Those trousers suit you" means (according to the Oxford Dictionary because I couldn't define it myself) "Enhance the features, figure, or character of (someone)"
Although one could possibly use "to fit" in terms of when something is convenient, it still requires an object and doesn't really apply to clothes. Although I just realised didn't actually read the first question and yes, "at passe" can mean both "fit" and "suit", I was just referring to this specific example :P
I would translate them as follows:
Do the pants fit me? = Passer bukserne mig? (size, shape)
Do the pants suit me? = Passer bukserne til mig? (how they look)