You can tell, sometimes rats get sores or their breathing is different and more raspy almost. There's tons of signs rat-lovers can pick up on for many different things. My best best best friend had three rats, two of them died. The first one to pass was the most loveable little mush of a thing, he was chubby and cuddley and just great, then a hairless girl who had a tumor who was also a sweety as well as a bitch. I know how you feel babe, when those two died I felt like they were my neice and nephew for a lil bit. It'll be okay, and maybe you can make room to save another rat (pet stores like pets'r'us treat rats like SHIT, give one a home)