The only way I know of to get a good measurement on my own would be to get a cheap bike and ride it up a steep hill in its lowest gear and time myself and measure the height of the hill. Hill height and speed are easier to measure than air resistance, though on a steep hill all forms of drag can have a 10% effect. There are online calculators for this.
According to my math, and evenly sloped hill of just 106 tall is sufficient to get a good reading of power for a minute. Once you know that value, you can compare it to the meter in the gym for your maximal effort of the same time length. You can then use that to calibrate the gym bike for longer distances.
400 watts = 400 N m/s = 400/9.8 kg m/s = 40.8*2.2 lb m/s = 89.8*3.25 lb ft/s, @ 60s = 17510 lb ft, divided by (143 lb rider + 20 lb bike) = 17510/163 ft = 107 ft vertically.
If you take Baseline Rd west to Central Ave, and head south, you get to the main entrance of South Mountain Park. Beautiful place with no entry fee. Nice ranger drove by me three times and asked once if I needed anything. Most fun work I've ever done. I took the main road in to Piedras Grandes. I downloaded the topographical map and found a good section that changed 100 feet in elevation pretty consistently and straight enough not to slow me too much.
The bike at Gold's Gym says I output 368 watts for 60 seconds.
The bike at a Scottsdale gym says I can output 520 watts.
According to the 20 foot contour intervals on that map, I can output 400 watts.
I'm disappointed, but it is better than being surprised when it counts.