When buying a secondhand cement mixer, note its brand and age. A mixer takes a lot of abuse over the years, so its age can indicate how much life is left in the mixer. Reading reviews is helpful as well.
Many sellers choose to advertise mixers for sale on Craigslist.org or in local classified ads. Check the mixer under consideration for cracks or rusted spots in the drum. Check cords to find any bare wires. In some cases, filling the barrel with some water to test for leaks may be worthwhile if you have doubts as to the soundness of the machine.