In level flight, if you know thrust, it's the same value. If you tow something in level flight, or strain gauge all the engine mounts, this can do it.
In gliding flight, if you can measure speed and descent rate (both fairly easy) you can calculate drag, since work done is rate of descent x mass x g, and it is also drag x TAS, so drag = Rod*M*g/TAS
(All in consistent scientific units of-course, most usually nowadays, SI, otherwise known as kg.m.S).
You might try
which many universities now use for the first couple of years of an aero-eng degree.
In reality, most aeroplanes will have a calculated drag value in use, based upon some combination of empirical calculations, CFD, and wind tunnel testing. There's an organisation called ESDU who are probably the main supplier of drag calculation methods, and most aeronautical engineering students will spend a great deal of "quality" time with ESDU data sheets.
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