Emma,
Lots of times I have researched the older machines so that I know what I'm looking for. When I decided that I wanted a Pfaff that had the IDT(full time built in walking foot), I researched the different Pfaff models that came with that feature.
I also ask questions of others as to what they like and don't like about a particular model of machine.
To be sure it works, I test drive it if possible with my fabrics and threads.
www.patternreview.com is a good place to start. You will find a section that has reviews of all sorts of sewing machines.