I'm looking to buy a 2nd hand road bike - what are some things I should look out for?
How do i know that I'm not just buying a low quality bike that will break after a year?
In addition, how do I tell whether the frame is in good condition?
Complex question. It helps to have some general familiarity with what is good equipment from yesteryear. The age of the bike if not known can be estimated from the gearing, 5-6speed rear cassette or freewheel suggests either more than 20yrs old or lower end 15-20yr old bike. 7speed pushes it into the 12-18yr range and 8speed was introduced in the early to mid '90s. 9spd popped up 6-8yrs ago. Exact timing can be given by more knowledgeable geeks but there is always an overlap of 2-3yrs as the newer equipment filters down the scale from topend to bottom. 7spd cassettes are on most of the Walmart $80 bikes sold now. The older the bike the more concern about stuff that wears out: chain is easy to check with a 12" ruler calibrated in 1/16". Cables rust and fray inside rusted housings, brake shoes change to concrete, plastic ossifies, grease turns to wax, tires rot, steel frames rust hopefully from the outside in, but also from inside out. Cogs front and rear wear, a worn chain suggests a worn cassette/freewheel and maybe even chainwheels. Refurbish here can be big $ even for DIY. Before even considering the previous, the obvious question is does the bike fit you: you have to know your preferred size. www.coloradocycling.com and other major sites have bike sizing criteria for straight and slope top tube bikes. So the bike is the right size, not rusted, rotting or obviously hazardous. Frame inspection for rust, paint wrinkles, bulges, dents and cracks should be done. Clamp the brakes tight and push the bike forward: does the fork wobble? could just be loose, or worse. Are the fork blades in line with the head tube? Eyeball the the bike from the rear: is the R der hanging vertical, not bent (usually inward), do the front and rear wheels line up?
Getting into the frame type (steel/Al/carbon (monocque versus lugged and glued)/ Ti : practically speaking Al will be by far the most common in bikes less than 15yrs old, steel predominant in older bikes. The others will be relatively uncommon. Carbon tubes glued into lugs were higher end bikes but the glues tended to fail and the tubes delaminated so such a frame should be carefully examined. Most riders liked the way they rode. Probably should be avoided. Al frames tend to be a bit stiffer with oversized tubing being the reason. They are a bit more likely to break but this is uncommon in road bikes.
Most difficult is the bits: Shimano is rampant on US sold bikes, Campy a bit player on road bikes in US
SRAM sells very little road stuff, mostly crossover type shifting/der for bikes that are in the comfort or touring category for light trail and mostly road riding. Older bikes can have a variety of manufacturers that no longer sell much in the US or are gone. No history lessons here.
Be aware that a lot of bikes were reviewed somewhere and a google on the bike name, if known, might well pop up a review especially for bikes sold in the past 10yrs. Here is an essay on used bikes that may be lower end than you want: http://bicycleuniverse.info/eqp/usedbikes-guide.html
Other posters may benefit from knowing what you want the bike for and how much you want to spend.