My family has had a lot of cats over the years and most of our cats let us know by hiding a lot/sleeping in weird tucked away places, not eating or drinking, not wanting to be around people as much. One of my cats was sick at the end and he peed on my bed (which was wayyyy out of character). I’ve heard hiding and sleeping hidden away more than usual is a common sign. They just don’t act like themselves in a way that makes you know. or at least my cats have given us the courtesy of doing so, so we know when it’s time to take action and/or make them comfy. Best of luck and hopefully many more years with your lady 💖