"How, then, do you tell if a tune is in a pentatonic scale?"

I have heard (and read) more than one definition of 'pentatonic.'

1. Any five notes you care to name.

2. The black notes, most easily seen on a piano.

3. The first five notes of a major scale, minus any sharps or flats that may be in it. (This seems to be for church bells.)
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I don't think there is such a thing as a blues scale. My dictionary says that a blues song has:

1. duple rhythm

2. uncertainty of scale between the major and minor third. (For example, a song in C might have an E, the major third *, in one place and an E-flat, the minor third, in another.

3. similar uncertainty may occur for the seventh note of the scale.

So if a song uses C D E E-flat, F, G A, B, and B-flat, we'd be hard put to say that it is using a certain scale.

* a blue note is an unexpected minor interval in an otherwise major song.
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4. the final characteristic is syncopation.

For me, the main characteristic of the blues is the creative use of half-steps, usually lowered half-steps, to create a mood of sadness. I think this fits pretty well with the dictionary definition.