How to tell if brake pads/rotor overheating?

140 isn't a bad temperature. What you need to watch for is the pads dragging due to a sticking caliper or piston. The easy way to do that is with a non contact thermometer. Take the temperature at each corner on the hub, brake rotor and caliper. Now go for a drive but try not to use the brakes and coast to a stop. Put the car in park and measure the temperatures again. You want to see an even but slight increase at each corner because the bearings and moving parts do create some friction. So say the car has been setting for a day the parts should be ambient temperature of say 75 degrees. After the drive you measure them and see all 4 corners are at 90 degrees, your good. However say three are at 90 but one is at 150 degrees, you will want to look it over because something is dragging.
As for overall brake temperature, it will vary a lot as the brakes slow you down by turning the motion into heat energy using friction. The faster the car is going when you use the brakes the hotter they will get. It also depends on how hard and fast you stop, the hard the brakes are applied the hotter they will get. They are designed for this though. The clue to overheated brakes isn't so much temperature as it is color. On a street car the rotors should always have a shiny silver color over the braking surface, if you see patches of blue or black the rotors have gotten very hot and you may want to check everything out. The next indicator is smell, some cheaper pads will stink if they get even mildly warm, but most quality ones will only start to really reek if they get very hot and you will see smoke from the brakes. That is when you may want to worry. That amount of heat will carbonize the grease in the bearings and boil the brake fluid.