So are you needing this for Linux or something else?
I'm afraid that the solution is very dependant on the OS, as there is absolutely no standardized support for this (unless there is some ACPI interface that does this - I'm not that sure about ACPI functionality).
The low-level parts of the graphics driver can probaby tell you if the monitor is in power-save mode or not, but whether it is switched on at the power-switch or not would require trying to talk to the monitor via EDID/DDC or some such. Whether the power-switch on the monitor stops EDID/DDC or not is something that probably depends on each monitor manufacturer.
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Mats