If you are a beginner, with less than several years of lessons with a qualified teacher, it is difficult to "test drive" a new flute effectively. Have your teacher or an experienced player help you. Have him or her play the new flute along with a selection of other flutes, including his or her "main" flute, comparing the sound on all pitches and on several differing pieces of music. Try to characterize the sonic differences -- start to develop your own vocabulary for expressing these differences.
Here are a few specifics you should make sure to test:
Ro -- Can it honk? How much dynamic range is available? How low can you get a ro meri?
Otsu register -- All notes in tune with minimal adjustment? How consistent is the volume between notes? How's the volume on meris and dai meris? Can you get full meris without any finger shading? What's the overall timbre compared with known flute? (Helpful vocabulary: focused v. unfocused, penetrating v. mellow, airy v. pure, strong fundamental v. strong higher overtones, secure v. breakable, simple v. complex... Please note that NONE of these are value judgments per se, it's the mix of elements that matters.)
Kan no ro -- How does the tuning of this compare with otsu no ro? How about i vs. ro -- same pitch without too much head adjustment? How's the dynamic range?
Kan register -- All notes in tune with minimal adjustment? How consistent is the volume? How's the volume on meris and dai meris, especially tsu meri and tsu dai meri? Can you get full meris without any finger shading? What's the overall timbre compared with known flute? What's the timbre like compared with otsu register? (Please see vocabulary above.) Can you play a ru (ichi san no u) with hole one partially covered? Do the following effects work well: ka ra, koro koro? Try ataris of re on holes one, two, three, four. How easy is it to control dynamics of hi and go no hi? How easy is a hi/go no hi trill? Can you get a variety of multiphonics? How's the "response time" on jumps from otsu no ro to high notes such as san no u?
Dai kan register -- How much do you have to lower your head to get an in-tune go no ha? Can you get at least as high as re fairly easily? How hard is it to "hold back" high notes?
General -- Do you feel the flute vibrate when you play a note, especially otsu no ro? Is the flute light enough that it won't cause you wrist problems? What's the "crack history" of the flute? Based on the maker's reputation and the cost of the flute, what's its resale potential?
Please remember that none of the above criteria are "show stoppers." You should aim for the balance of all these particulars that's right for you.