Yes, looking at the key signature is a good and efficient way to make sure of your key-note! Also have a look where the clef centres: "Belle qui tiens ma vie" is a good one to try this out on, as you have four lines to look at (the SATB lines start on G, C Bb and G). In the case of Les Bouffons it's a straightforward G-clef i.e. the common-or-garden treble clef, with the tune starting on an F.

If you're happy with the F naturals in Branle des Chevaux, well done! VERY rough rule of thumb is that sharpened leading notes crept into later music, i.e. F natural is in a sense 'earlier' than F#. (Don't take that as any sort of hard and fast rule, though, as it certainly isn't!)

With a foot in both 'folk' and 'art' camps, I find it fascinating that the difference between accidentals is much smaller in folk than art music. Many a folksinger have I heard treat a major or minor third (say) as colouring only, whereas in art-song one is quite often be required to make the difference crystal-clear, perhaps with emphasis and certainly with micro-tuning (the major extra-high, and the minor pushed right down into the note).