Just as a matter of interest, what would everybody think about using the opening bar as check-guide *after you've worked it out from the accidentals etc*? (Not the "end" bar of an extract in these instances because in the exercises it's often open-ended so it would be less reliable.)
In Exercise 34 of the OP's book, for example, the keysig is one flat and you're asked to name the key. So it could be F Major or D Minor. The C has been sharpened by an accidental so you would choose D Minor. But it gives you a nice warm glow

when you see that the opening 3 notes of the extract are D-F-A, the tonic triad of D Minor so you can be even more certain that you've got it right.
It's not always as clear-cut as this so it can't always be used for reassurance. In the exercise the OP asked about, the second extract has Eb-F-G as the first 3 notes so it doesn't provide any reassurance at all because Eb and G appear in both C minor and Eb Major.
In the first extract though, the first 3 notes are G-C-C. This combination appears in the tonic triad of C minor but not Eb Major, so is it fair to say that it would be reassuring to think you'd got the right answer if you'd worked out from the accidentals that it was C minor.
Just wondered what everybody thought about using this "belt and braces" approach where it was possible to do so - for reassurance, as a double-check, not to work it out by using this approach. And if it didn't tally, it would give you a clue that you might have got it wrong and you could go back and work it out again.
NB. I haven't mentioned inversions etc because you don't get those at G3.