"How would one determine if a student got 1 minute of instruction, or 1 hour?"
By either the student or the instructor entering time in the "instruction received" column of the students logbook silly 
.
A signature confirms that training was done. It doesn't verify the authenticity of anything, just that training was done.
You stated that the sig doesn't verify the authenticity of the hours. If that is so, then in my mind, the DUAL column is meaningless, and it should just be a yes/no checkbox. By signing your name beside X hours, it means that you gave that much instruction.
Anyway, since you're the CFI, I'll defer to your experience. I guess I'll just have to be on the lookout for suspicious entries in the books I'm signing. Thanks for the info.