Great question — answering as the UphillTech team:
It's probably worth riding, if you don't have another bike. First, of course, simply wipe off the dust (or hose it down and wipe dry). Then examine for bad rust. If it's an inexpensive bike that was stored where it wasn't completely dry there's apt to be some rust spots here and there, which is OK. But gobs of rust around the bearings is a bad sign. If the chain is badly rusted that's not fatal, but not a great sign. Air up the tires. (Once you decide you'll be a bike rider you'll want to get a bike pump of the full-sized "floor" variety.) Then pick up the bike and spin each tire -- they should spin freely, with no gritty feel in the bearings. Note that the brakes may drag, but that's not a...
Full answer by Daniel R Hicks: https://bicycles.stackexchange.com/a/8919 (CC BY-SA 3.0).
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