1989 Buick Century 1989 Buick Century overheating issue

I have an 89 buick century with the 2.5L 4cyl engine. It is overheating and I noticed when it was hot that the radiator fan is not coming on. Is this a sensor issue, which was suggested by my local Napa, or is it a fan motor issue, also suggested by Napa? I am not all that savvy with auto repairs although I am fairly intuitive. If it is a sensor problem, where do I find the sensor on this make/model? Thank You in advance for your answer and I look forward to hearing back from you.

Ed