Um...you probably can't, not in the way you mean. Text files aren't "sent" to the printer anymore: they are printed via a printer driver to a device dependant format (which includes images, fonts, and so forth and depends on the printer manufacturer) to a spooler which feeds these formatted files to the printer itself (again, via a manufacturer specific driver).
The text file itself only "exists" until the initial print driver conversion is completed.
And to monitor the printer itself requires knowledge of the specific printer - and probably assitance from the manufacturer.
You can monitor the printer spooler queue, if that helps:
Monitor jobs in a printer queue (.NET)[
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Or there is the API directly:
https://msdn.microsoft.com/en-us/library/windows/desktop/dd162861(v=vs.85).aspx[
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