Bob, your question certainly is valid, especially on the newest trucks.
Just because your flashers work does not necessarly mean your brake lights are working.
An example would be the newer Super Duty's. There is a circuit, fuse and a relay for just about everything in the truck. Including separate relays for left, right, run and brake lights for the trailer connection.
For the most part, if you can see your brake lights and hazards from the truck reflecting at the front of the trailer, the possibility is that if the hazards work at the back of the trailer the brake lights work too.
But, that is not 100%, so the only way to really tell is to have the back of the trailer close enough to an object to see the lights reflect.
And as was mentioned, if you have a dash indicator that is blinking faster then normal that is an indication you have a bulb out.
The caveat to that is the "fast blink" is set off by a resistance change in the circuit, e.g. one bulb out, less resistance, fast blink.
If you are connected to your trailer and the truck has a bad bulb, you may not get the fast blink because of the added load the trialer supplies.
Same goes with if you have a bad bulb in the trailer. The system already has sufficient resistance so the fast blink does not activate.
LED's are now a game changer. Because they are so low in resistance, body computers have to be modified to operate the lights correctly.
My motorcyle for example has to have a "load equalizer" in the system to allow my lights to work correctly. Not only will my lights not work correctly, but the anti theft portion of the PCM will not work with the alarm with straight LEDs