If your wife is a violinist there are many things than can be transposed from violin to cello in the search for quality. Don't forget the bow.
There are so many things... I will try to make a real answer in the coming hours and days by editing this one, I feel I have not yet touched 1/10th of what should be given.
But first, have you considered renting? Many good instrument shop propose this option, sometimes with a refund if you buy the instrument you rented after a while. Under a deposit and ID they can sometimes allow you to take an instrument or a bow for a few days to have it tested by a friend.
If you rent, you will want something already good to practice, without flaw in setup. If you buy, you might accept to take the risk of having an instrument that will evolve with you and to have it set up or revised.
If you do not play yet the instrument, ask someone from the shop to demonstrate it to you.
A general advice for all instruments buyers that is common sense but is worth repeating, go with a knowledgeable friend or good contact such as a cello teacher, someone with several years (and several instruments) of practice. Their experience (and previous mistakes) can be very valuable, and they can play the instrument for you to decide. Many things I talk about require to be able to play the instrument to test really.
Quality of craftmanship
- A first good thing to check is the purfling, visually: the inlays should be genuine, three layers : normal wood sandwiched in two layers of black wood. You can spot that because the wood in the purfling should have a varying and different orientation from the table or back wood figures (see the picture). Low quality cello (and violins, violas) have sometimes painted or otherwise faked purfling.
varnish application : look were it is difficult to apply it evenly : under the fretboard, near the neck, on the transition between the sides and the table or the back. You should see the grain of the wood.
Carved wood. Some very cheap cello tables or backs are thin wood slices pressed to form instead of massive blocks of wood carved to the final shape.
cut of the saddle : should be very neat, without level difference with the ridge of the table.
- Should not be too heavy. Good lutherie wood has dried for many years before being used, staying hard but loosing weight. At equivalent dimension and thickness good wood will be significantly lighter than too young wood.
- Look for any cracks, but especially on the middle of the left side of the table and the back, the so called "sound post" cracks.
SET UP Quality
Good sustain when plucking open strings
A too strong dampening of a string is a sign of a problematic set-up: incorrect diameter of notches in the nut or the bridge, incorrect lubrification of these, badly set sound-post, ...
Minimum level of sympathy between strings when bowing open chords (be sure that the instrument is properly tuned).
Evenness accross strings
Many people like this and look for this: the idea is that you should be able to play a melody across two strings without a noticeable change in timbre and sound apart from the frequency of the notes. To do that you have to be able to play at least in the middle positions of the cello (3-6) to compare different ways of playing the same notes on two different strings.