My current ride is a great thrift store find - a shimmery teal Made in Canada steel bicycle in fabulous condition for $4.99! When I got her last summer I was super happy cause I hadn't had a bike in months since the one before was stolen. Just some quick maintenance and air in the tires and she was good to go! How did I know it was a great bike and what did I do when I brought her home? I've found and lost, worked on and broken so many old bikes I've developed a system for when I first find one, mostly to stop myself from bringing home more greasy metal than my apartment can handle!
Finding the bike: be it a thrift store, garage sale or ditch find I only bring home bikes that can be fixed up with the simple maintenance steps I list later in this post. Anything requiring more work than that isn't worth it (for me anyways, bike nerds - go nuts!) First thing I check is simple: make sure all the key parts are there: frame, 2 wheels, gears, sprockets, chains, handlebar, pedals, brakes. All the parts should be in good condition- check for scratches, dents and rust damage but seized chains or small piece replacements are pretty easy and rust can be removed depending on your determination! Check the comfort and condition of the handlebars and seat, although these can be replaced, its better if they don't have to be. Make sure nothing is embedded in the tires themselves, but a bike pump or new tubes is a small investment if the tires are flat. If you're feeling particularly investigative - check the frame to see what the bike is made of and where it was made. As a bonus its nice to see if the bike has any special vintage features such as fenders, lights or even a licence plate!
Tuning it up: When I bring a bike home for the first time I do a few simple things. Start by wiping down the bike with a soapy rag then clean soft cloth. Use a soapy toothbrush to clean out any bad spots and steel wool for major rust buildups. Clean the chain, gears, shifters, deraileur and all moving parts of the drive train with WD-40 then lubricate with a bike specific waterproof chain oil. Inspect the brake handles, lines and brake pads and lubricate if necessary. Use a pressure gauge if you have one otherwise sit on the bike to check the pressure in the tires. Replace the tubes if necessary and use a bike pump to keep them well inflated. This will help reduce wear and tear and make it easier to ride!
Caveat - I don't claim to be a bike expert by any means. This advice is simply what I've learned to do by some internet research and lots of profanity inducing trial-and-error over the last number of years.