When you buy old bike

CHECKLIST OF WHAT'S LISTED BELOW:
    (Not a substitute for reading the whole thing, but
     useful for quick reference at a seller's house.)

• FIRST THINGS FIRST
  • Don't buy the first bike you see. Bring a friend & a flashlight.
  • Do a couple of practice inspections on friends' bikes, discuss the results.
• OVERALL APPEARANCE
  • Is the bike clean and straight? Sight down centerline, and down forks.
• HAS IT BEEN CRASHED?
  • Check for bends or scrapes on bars, exhaust, plastic, and levers.
  • Short/shallow/non-parallel scratches/chips ~= tip-over.
  • Long/deep/parallel scratches and cracks ~= crash.
• HAS IT BEEN RACED/ABUSED?
  • Check for small (1/16") safety-wire holes in bolts. (Particularly caliper mounting bolts, exhaust bolts, etc.)
• BIKE-SPECIFIC
  • Know the characteristic flaws of the models you're going to see.
• BRAKES
  • Check for smooth operation, no pulsing, pad material remaining, etc.
• CLUTCH
  • Check lever effort, and whether the clutch releases when squeezed.
• GAS TANK
  • Look for rust or a milky paint-like coating on the inside.
  • Dark gas (tea colored) is an indication of old gas that needs changing.
• SEAT
  • Look for cracks/tears/etc.
• TIRES
  • Check: remaining tread depth, dry rot, profile (round? squared-off?), date code.
• ELECTRICAL & BATTERY
  • Test all lights and switches to make sure they work.
  • The sound of the starter cranking is a decent meter of the battery's condition.
• SUSPENSION
  • Check forks for seal leaks, scratches/nicks/bends/twists in legs.
• WHEELS
  • Check both sides of both wheels for dents/cracks.
• CHAIN/SPROCKETS
  • Check for chain/sprocket wear (hooked teeth, stretched chain).
• EXHAUST
  • Scratches/rust/damage. Exhaust pressure equal on both sides?
• ENGINE/FLUIDS/CARBURATORS
  • Check starting and operation of engine and carbs. Check for leaks.
• CENTERSTAND CHECKS
  • Get wheels in air and check wheel bearings, brake operation, etc.
• SERVICE
  • Service records available? Proof of warranty work? Etc.
• DIRT BIKES
  • Special concerns when inspecting dirt bikes.
• QUIZZING THE SELLER
  • Questions to ask the seller to determine the condition of the bike.
• ACCESSORIES, PRICE, and DEALING
  • Are you willing to pay more for add-ons?
• HELMETS
  • Used helmets are worthless. Don't use 'em, don't pay more for 'em.
• TITLES & PAPERWORK
  • Make sure it's clean & that the VIN numbers match up.
• TEST RIDE
  • Go on one if you can -- you can learn a lot about a bike this way!
  • Make sure you give it a pre-ride check to make sure it's safe to ride!
• AFTER THE PURCHASE
  • Some tips after you get your bike.
• INFO FOR NEW RIDERS
  • Not the biggest and the baddest -- start with something easy to control.
• INSPECTION PICTURES
  • To aid inspection, some pictures (and, eventually, diagrams) of what certain components look like.
• RECOMMENDED READING
  • An exceedingly incomplete list of books that'll teach you more about bikes.
• OTHER WEB SITES
  • A listing of other motorcycle-related web sites with useful tips for bikerscum. 
  Source :- http://www.clarity.net/adam/buying-bike.html