I had been a Windows guy for a decade and I am planning to buy a used Macbook Pro (2009 model). What should I look for and check when buying a used Macbook Pro?
Beyond visual inspection, you should be able to run Apple Hardware Test in order to check the system hardware's working condition.
Besides the hardware test, it's very important to know the health of the battery! Although the results aren't 100% accurate, coconutBattery gives you a fairly good result about how many cycles the battery has had. If the number is over 300, you can almost be sure the battery capacity is lower than 80%. But you should ask the guy you're buying from how long the battery lasts and if you don't trust him, try it yourself.
A more obvious question is if the Mac is still under warranty. Since it's a 2009 notebook it's very unlikely, but maybe it has Apple Care... You never know.
And the last thing I would check, besides the things already mentioned by AJ., is the optical drive. Does it read/write CD/DVD's at an acceptable speed.
Look for future OS X support
Apple is well known for dropping support for older hardware as time goes on. The pending Mountain Lion Release is a perfect case in point.
Here is an extract from the linked page:
Macs that will support OS X 10.8 Mountain Lion
As usual, the newer the Mac the better:
MacBook Pro – 13″ from mid 2009 or later, 15″ from late 2007 and newer, 17″ from late 2007 and newer
MacBook Air – late 2008 and newer
iMac – models from mid 2007 and newer
MacBook – 13″ aluminum from 2008, 13″ from 2009 and newer
Mac Mini – early 2009 and newer
Mac Pro – early 2008 models and newer
XServe – early 2009 models and newer
Macs that are NOT expected to support OS X Mountain Lion
Older Macs and those with weaker GPU’s will likely be left behind:
Anything with an Intel GMA 950 or x3100 integrated graphics card
Anything with an ATI Radeon X1600
MacBook models released prior to 2008
Mac Mini released prior to 2007
iMac models released prior to 2007
Original MacBook Air
First thing to check with a secondhand Mac purchase is the physical condition. This can speak to the owner's treatment of the device. Scratches, dents, dust, sticky marks, food crumbs, etc. Check that the screen lid closes squarely onto the lower section. Any warping or bulging of the case is a 'No'. Display problems, like pink lines, is also a red flag.
If you can get some sense of whether the owner is a 'Mac-head', that might be a good sign. There are steps that the owner must complete before selling a Mac, like signing it out of their AppleID, so you need to make sure they have done these tasks correctly.
Next: How old is it? Does it support the latest OS; and if not, how much life will you get out of it on an older OS? Even if it does support the latest OS, is it likely to get cut-off in the near future?
Battery degradation with age is to be expected. You may want to factor the cost of a battery replacement into your total investment. Over 1000 cycles is bad; but few cycles is no guarantee either: a 5-year-old battery with only 260 cycles might still hold less than 80% charge or be unreliable.
There are Diagnostic tests you can run on the hardware; though in my experience, Macs tend to be fine right up until they aren't.
Spec is obviously critical: a decent CPU (stay away from the low-core ones) with decent RAM and storage.
Only you can answer the questions "How long do I expect to get from it?" and "How much am I prepared to pay?" Do you think you can sell it on after you're done with it?
Macs generally hold good value on the secondhand market. At the time of writing, I suspect the last Intel laptops (2016 - 2020) will become very cheap, as they have a few design flaws (butterfly keyboard; overheating); and the move to Apple Silicon will render them yesterday's news. So, conversely, this makes them a good bargain.