HP AC adapter, HP part number 416421-021?http://www.amazon.com/Compatible-HP-416421-021-Adapter-Charger/dp/B0050DN8X8
Plug going from AC adapter to DC Power Jack on laptop;
Center of the plug is the Positive ( + ) connection.
This is where the red Positive probe lead of the multimeter goes.
The outside cylindrical metal shell is the Negative ( - ) connection.
This is where the black Negative probe lead of the multimeter goes.
The Function knob should be set to DC Voltage.
(Unless you have a multimeter, that automatically detects whether it is AC, or DC voltage you are testing)
If more than one scale set it to the 0 - 50 Volt scale.
You should read VERY close to 19 Volts.
[Label on AC adapter.
Output: 19V / 4.74A
19 Volts at 4.74 Amp's. (Amperes)
Don't worry about the Amperage, just the Voltage ]
For additional questions please post in a Comment.