How do you know if you have a cavity?

Just in the last couple of days, when I brush my teeth on my back tooth it hurts to brush. I brush my teeth twice a day and floss once a day... and that tooth really isn't bothered by anything else. I am also thinking it might be my wisdom teeth. I got sealants for wisdom teeth (i think? i just know i got sealants, idk what they were for) about 3 months ago. I am going to the dentist next week for the regular cleaning, but I was wondering if there is any possibility that this could be a cavity. Thank you! :)