Measuring it won't blow the boards, it's when you try to use it.
as far as the led your talking about the 220 ohm is in series with the (led) you could even run your led from track voltage with the 220 ohm resister in series!
Running a standard white LED from track voltage with 220 ohms in series is running it at about twice it's rated current, the LED will NOT last long with that configuration. If you assume half-wave RMS value for the voltage applied, for 18VAC, that's 12.73 RMS volts. Assuming the white LED operates at around 3 volts, that gives you 41ma RMS to the LED. Next, let's talk about the reverse voltage specification of the LED component, it's typically around 5V. Peak voltage for a half-wave 18V is 12.7 volts, far in excess of the rating of the LED.
If you want to run LED's from track voltage I recommend 470 ohms (or larger) and a diode to protect against reverse voltage. If you have two LED's, you can wire them back to back with the polarities reversed, they protect each other and you only need one resistor and no diode.