>> Er, the firing order is not 1-2-3-4 normally, it's 1-3-2-4 or such and
>> it depends on the engine!!!. Be careful ;-) If you have more than 4
>> cylinders the firing order gets complicated (there can be several
>> combinations). Get a manual or 'consult' another car of the same type
>> if in doubt.

1342 on most I have seen (but not all).

When all else fails.

Take out plugs
Take off rocker cover.
Turn engine (manually or electric inching) until exhaust valve(s) open on 
one cylinder.
Some other cylinder will have both valves shut (compression top). This will 
be opposite of the first cylinder in the sequence.
If desired check that piston is at tdc with a screwdriver (NOT a nail etc 
:-))(whoops, where did THAT go?!)
move motor on until another has valves close and piston at TDC.
Continue until all done or pattern obvious.

The ultra keen and clever may look at the cam profiles and work out the 
firing order from them.
(No engine turning needed, error easy).

Bearing in mind the (probable) crank throw pattern will limit the 
possibilities. Beware of eg 5 cylinder Audis :-).

        RM 

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