It's easy enough to tell a German-fingered recorder - once you've paid for it and had it shipped to you. Better to avoid the possibility.

It's harder to tell a mistuned Baroque-fingered instrument. They're most likely to be off on the high A, so check that first. Some recorders may have an idiosyncratic B and C sharp - this can be worked around, but you'll need different fingerings than the standard ones if they're to be in tune with other people.

The G sharp/A flat in the lower register is so variable that if you aren't using the same model of recorder for the whole group, you'll have to work out the right fingering for each instrument individually. (For players who know what they're doing this is a feature, not a bug - gives you the opportunity to tune these notes close to meantone).