@ Kerry, This part of the link isn't correct:
"Test both outlets because they can be wired separately. The fact that one is properly wired does not guarantee that they are both properly wired."
There may be some special outlets where that is possible but not standard outlets.
If there are two seperate circuit house wires at an outlet which would also be non-standard . If you have black on the same side as white on the top & bottom outlet you will have a short. This will blow the circuit breaker.
The common situation is one pair of wires B/W is powered to the outlet.
The second set run to the next outlet in the same circuit to power it.
If you remove that pair the next outet will be "dead".
Here is how to test with a multi-meter.
Set it on the correct AC voltage .
Test between the small slot & the U shaped ground slot.
If you have house voltage. Between 115 & 125 then the outlet is wired correctly.
Do the same between the long slot & the U shaped ground slot. You should have no or very little voltage . If that is what you find. You can test between the long slot & the U shaped ground slot for continuity. That isn't necessary but there should be continuity.