0ne of the best purchases I ever made was a Infrared red heat gun ( Amazon , $22.00) . I take it with me when towing my boat , I take it out when towing and get an exact heat reading of the tires and the hubs . I get a baseline reading when I know my hubs are well greased and running smooth --then any heat increase of significance can be attributed to early bearing wear .
It also gives me a decent indication if one of the tires is wearing " odd" and heating up .
It might not be perfect , but it gives me an idea of a "hot " bearing or tire .