There's a simple way to know when one of your resistors has gone bad [btw, there are two of them]. They are ceramic and when one of them no longer operates the other bears all the electrical load...it overheats. So what happens is you'll hear your low speed fans run until the resistor gets too hot [car on or off], then they will start running fast until the resistor has a chance to cool down. About 10 seconds later you'll get your low speed fan back but once the resistor gets too hot again you'll get high speed fans...basically, fan 'cycling.' You can tell which one is GOOD by feeling it and watching your skin melt off from the extreme heat.
OR you can measure resistance across with a multimeter, this should show a value between 0.1 and 0.9 ohms if it's still working.