How could you tell that the cervical mucous is changing?

Consistency. Mucous changes consistency. It can become more thick and stringy or thin and watery. Thicker mucous mid-cycle is typical of ovulation.
Best way at home. You have to feel confortable with placing your finger inside the vaginal and touching the cervical opening and seeing if the mucus is thick or stringy. Around the time of ovulation it is thin, almost watery and when you touch a qtip to it and pull away it is quite stingy. If the office we also evaluate it under the microscope for ferning.