heavy or labored breathing. heck, just being able to hear them breathe. a snotty nose and/or runny eyes in combination with the heavy breathing and/or lethargy.
to prevent it get them out of the tank and into a much better ventilated cage. the ammonia builds up VERY fast in a tank of any size and that will damage their lungs and lower their immune systems making them more susceptible to illness if there is no way for them to get away from it. having them on a good diet that will help keep their immune systems up. keeping them active and mentally stimulated as that also increases their immune systems.
to stop it you'll need a good rat vet not merely a vet that deals with rats. you want one that is knowledgeable and experienced with rats. he needs to be able to recognize the early signs of illness and know or at least willing to look up immediately the medications that can be used in a stubborn case as well as a simple one. and you'll want some repor with them even before the rat gets sick. this will make it easier if you need to get a payment plan installed as they will know and trust you. you also will not want to delay in getting them vet care. tumors and respiratory issues are the leading cause of death in rats. the sooner you catch it the better chances your rat will have. this is another reason why it good to have a vet already lined up instead of waiting until your rat is sick and searching the phone book.