The most certain method is to measure battery voltage, with the accessory plugged in.  The bike should be able to hold 13V at anything over idle, with nothing connected.  With your accessory turned on, rev the bike up to your normal highway cruising RPM, and see if the voltage is above 13V.  If it is, you are good for the highway.  Now bring the revs down to half the cruising revs on the street, and see if it can still hold 13V or more.  If so, you're safe on the street.  If you can't hold 13V, you will need to play it by ear- measure your battery voltage after each ride for a while, with the ignition on, but the motor off.  Write down the numbers and dates.  If you see voltage falling day by day, then you have to keep the revs up more, or use the accessory less.