When looking at timecode, how can I tell drop frame from non drop frame?

When looking at timecode, how can I tell drop frame from non drop frame?:
This questions used to get me every time until someone (I can’t remember who or I would give credit) told me drop frame is a semicolon because you “dropped” a period. (You dropped the period and have a comma instead, get it?)  Meaning non drop frame could only be a colon.

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