Evaluating an aircraft for potential purchas is fairly "easy." It just takes all day or two. Obviously, go over the logbooks for continuity of service (no unexplained gaps). Incomplete logs, or "reconstructed" logs are show-stoppers and reason enough to walk away.
While perusing the logbooks, I look for repetitive write-ups that always seem to get squawked but never seem to get fixed.
Verify the total airframe and engine times, and make sure they agree with any (non-binding) "status sheet" that the seller has generously provided. Other obvious areas: Last annual/hundred hour. (Do not take the seller's word on anything. They lie. Alternately, they might simply be too dumb to know what's important and what's not. *YOU* must be smarter than them.)
Check the component historical record cards of EVERY life-limited component, airframe *and* engine. Failure to do this will be at your peril. Remember, if it's a life-limited component and does *not* have a historical record card and you cannot accurately and legitimately verify the total number of hours on that part, then it's not an aircraft part, period.
Other components that have overhaul intervals must also be checked for compliance (by cross-checking the "yellow tag" against the logbook entry for that component's installation and calculating the time in service. FOR EACH COMPONENT.
It is best to have a computer spreadsheet program that will allow *you* to plug in numbers and determine remaining life limits. If the seller provides one, MAKE SURE that they don't "conveniently" leave off some critical part that just coincidentally has very little time left.
Check for compliance of every applicable A.D. (airworthiness directive) or Service Bulletin.
Then check that all obvious repairs or alterations to the aircraft have been properly documented either by logbook entry or other acceptable means (e.g. FAA 337 form). Check the equipment list to make sure that everything installed is actually *on* the equipment list and conversely, what is on the list is in the ship. Check the latest aircraft empty weight and the most recent weight and balance.
Finally, check that everything actually works - comm and nav radios and the intercom system. Don't overlook stuff like the instrument lights! Sometimes helicopters don't do a lot of night flying. "Little" things like inoperable instrument lights might go "unnoticed"...until it's *you* up there on your first night flight and you turn the instrument lights on and go, "What the..."
If the ship is old (L-3 say), a really thorough evaluation can take the better part of two days. Don't be in a hurry. Be thorough and do your due diligence. This is the only way you can be reasonable assured of not having any post-sale "gotchas." It is VERY EASY to get burned.
If you're evaluating the ship for lease, then the pre-purchase inspection doesn't have to be so thorough. Verify the legality of the aircraft, and let the principals work out who pays for what, component and maintenance-wise.