Determining the correct polarity of a supposedly wrong polarized capacitor
That's funny, they actually added two plusses to the screen and they still put it in backwards. 

You could power it up and see which pin is more positive, but powering it up without the cap may also affect the results.

You would be better to lightly solder in the cap the way it was, since it was working, leaving the legs long in the direction you think is right then power up and check the voltage is positive on the positive. Then power off and attach the cap properly, reversed if need be.