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If the monitor does not have a symbol (similar to the letter 'C' with a dot in the middle), the cheapest digital multimeter can be used on the 'diode' or 'continuity' setting to check the input.
If the meter reads between 600 and 900, you are measuring the forward voltage drop of a protective diode.
Any other reading will mean that the positive probe of the meter is on the positive part of the connector.
i.e. If you are placing the positive meter lead on the center pin and the negative on the shell and it reads 600-900, then the power supply needs to have the negative applied to the center pin and the positive on the shell.
If the negative lead is held on the center pin and the positive on the shell and the reading is 600-900 then the center pin is the positive input and the shell the negative.
The readings shown above mean that you are measuring the forward voltage drop of a diode meant to protect the unit against improper polarization and it will probably either die or ruin the adaptor if misapplying the polarity.
You can buy such a meter at electronics or auto parts stores for under $20 US.