Diagnosing motorcycle ills is a relatively simple process if you use orderly procedures and keep a few basic principles in mind.
Never assume anything.  Don't overlook the obvious.  If you are riding along and the bike suddenly quits, check the easiest, most accessible problem spots first.  Is there gasoline in the tank?  Is the gas petcock in the "on" or "reserve" position?  Has the spark plug wire fallen off?  On Pioneer models, check that the ignition switch is turned on.  Sometimes the weight of keys on a key ring may turn the ignition off suddenly.
If nothing obvious turns up in a cursory check, look a little farther.  Learning to recognize and describe symptoms will make repairs easier for you or for a mechanic at the shop.

Describe problems accurately and fully.  Saying that "it won't run" isn't the same as saying "it quit on the highway at high speed and wouldn't start" or that "it sat in my garage for three months and then wouldn't start."
Gather as many symptoms together as possible to aid diagnosis.  Note whether the engine lost power gradually or all at once, what color smoke (if any) came from the exhausts and so on;.  Remember that the more complicated a machine is, the easier it is to troubleshoot because symptoms points to specific problems.

You don't need fancy equipment or complicated test gear to determine whether repairs can be attempted at home.  A few simple checks could save a large repair bill and time lost while the bike sits in a dealer's service department.  On the other hand, be realistic and don't attempt repairs beyond your abilities.  Service departments tend to charge heavily for putting together a disassembled engine that may have been abused.  Some won't even take on such a job­so use common sense and don't get in over your head.

OPERATING REQUIREMENTS

An engine needs three basics to run properly: correct gas/air mixture, compression, and a spark at the correct time.  If one or more of these components is missing the engine won't run.
The electrical system is the weakest link.  More problems result from electrical breakdowns than from any other source.  Keep that in mind before you begin tampering with carburetor adjustments and the like.
If the bike has been sitting for any length of time and refuses to start, check the battery (if so equipped) for a charged condition.

Look at the gasoline delivery system next.  Check the tank for fuel.  You can check the fuel lines, filters, and petcocks by disconnecting the fuel line at the carburetor and turning on the gas. There should be a steady flow.  If not, remove the petcocks from the tank and clean their filters in solvent.  Check the gas lines for obstructions.  Gasoline deteriorates with age, so if you suspect the machine has been sitting for quite a while, drain the gas and use a fresh supply.

Compression losses usually enter the picture only in cases of older machines.  Causes may be worn or broken rings or piston or a worn cylinder bore.  Usually a gradual power loss and harder starting will be readily apparent in this case.