I would like to check the polarity of the output of a DC wall transformer at the leads.
If I connect the output to an oscilliscope with the polarity reversed will the voltage appear on the screen as negative and not harm the oscilliscope?

 

That could depend on the voltage. But generally yes, a positive voltage swings the sweep upward while a negative swing sweeps downward. Make sure you set your scope correctly.

 

Oscilloscope inputs are difficult to destroy.

 

Why not just use a DMM?
That is the easy way.

 

...or use a diode
...or use an LED with a series resistor (three LEDs in series plus a 330 ohm resistor if a 12v supply).
...or hold it in some concentrated salt water and watch which side makes bubbles.

 

The supply probably has a molded on connector that salt water will ruin in short order. and certainly a voltmeter is the preferred choice for determining polarity.

 

I think MrSalts was joking about the salt water. If he wasn’t, he would surely havre said to do under a hood do you didn’t breathe in the chlorine fumes.

 

Don’t both sides make bubbles? Hydrogen at the cathode and chlorine at the anode?

 

Don’t both sides make bubbles? Hydrogen at the cathode and chlorine at the anode?

Yes and no - salt water makes Chlorine and sodium hydroxide (and hydrogen). The so-called Chlor-alkali process. The time it would take for someone to determine which wire made bubbles would generate much less chlorine gas than emitted when taking the cap off of a bleach bottle.
Just look for initial bubbles at the negative terminal (hydrogen). The positive terminal takes a while to make bubbles because the chlorine is soluble in water to some degree. Note, I've done this with a nickel sponge anode so the high surface area will dissolve the initial chlorine much better/faster. Both anode and cathode may generate gas if done with simple wires. A fun experiment. And there are certainly better ways to determine which end is positive.

 

The supply probably has a molded on connector that salt water will ruin in short order. and certainly a voltmeter is the preferred choice for determining polarity.

That's why one would connect the connector to a pair of wires that become a sacrificial electrode when dipped into the liquid.

 

No, salt water makes Chlorine and sodium hydroxide.

Don’t believe it, it’s a lye!