This is one of the hardest things to do. Every interaction with a possible new cello is colored by the sound of your current cello no matter if it is good or not. Most players first words about a trial instrument are not an unbiased review. Instead, they talk about the new instrument in relation to their old instrument, sometimes unknowingly.
If you have a student instrument, a great professional instrument will be a shockingly different thing. But many times, the brilliance or dynamics of a great instrument will be thought of as ugly to a student who has been playing a typically dark and muddy sounding instrument.
Then, if we understand that we may should like something that we find so strange to us, how can we possibly make a good decision? That is a question for the ages!
The logical answer is to find someone who has enough experience and a wide enough view of all the things a cello needs to be able to do to guide you. Oh, and you should be able to trust their opinion which generally means that they should not be able to profit from the purchase of the instrument in question. See “Whose Opinion to Trust?” for the best judges of instruments.