The key things for a wheeled backpack are the same as on a suitcase. The zips, wheels, material, handle and warranty
Zips – they should be rugged, large and lockable (on all pockets if that’s what you need)
Wheels – wheeled backpacks always have 2 wheels, so look for large wheels with a durable tread, preferably on an axis (avoid smaller roller blade wheels) and a generous kickplate and protective plastic housing around the base of the case. This will absorb the impact of being dragged upstairs and so on, rather than wearing through the fabric.
Material – ripstop nylon, water-resistant, thick and highly resilient.
Handle – the poles should be sturdy – a warped pole can mean a handle that stops going up and down so the poles should be sturdy and have adequate protection from getting damaged.
Warranty – the warranty is generally a good indicator of quality, 1-3 years would be sufficient for a shorter and less rugged trip, 5-10 years is ideal and lifetime means the company really backs their product – look for a lifetime if you really need something high quality (Osprey’s lifetime warranty is a good example!)
Pack comfort and support – if you are purchasing the wheeled backpack just so you can catch a train or get across the odd paddock then you can get away with something simpler and lighter, it should always have a lumbar or chest strap though if it’s over 50L. For anyone who will need to actually carry their pack, look for adjustable shoulder straps with great padding and comfort, test it out in the shop or before you go with the bag weighted to check the support.
Features such as a removable day pack, raincoat, mesh panels and so on are dependent on your specific needs for your trip.