How can you tell if the engine you have is in timing when you get it?
If you got it through a decent dealer, it will have been checked. If you didn't get it through those means.
Check that it is firing somehwere between 25 - 30 degrees BTDC. (28 is ideal but a couple of degrees either way is not going to make one iota of difference to the final outcome).
For a quick and dirty test
Find TDC by looking at the piston through the exhaust port. - mark a pencil line across the prop hub and the crankcase at this point.
Method one for finding the right firing point
Get a strip of paper and wrap it around the prop hub. Cut it so it is exactly the length of the hub circumference.
Get that piece of paper and cut it in half - discard one half.
Get the piece you kept and cut it in half again - discard one half.
Get the reamianing piece and cut it into a 1/3 length - discard the 2/3rds length.
That last peice you kept is now ''30 degrees'' long. Make a mark from the exisiting mark on the hub to the length of that paper before TDC (to the left of the mark looking from the front of the engine.
Method 2 for finding the firing point.
Measure the diameter of the prop hub and muliply it by 0.244 - that is the 28 degree length. use that measurement to amke the BTDC mark.
Now power up the ignition, and listen to the spark plug cap - it will make a ''crack'' noise when it fires. Do it slowly and don't overdo it. There is no need to have a spark plug in there for this test as the occassional use of it this way woun't hurt a thing.
Now you will most likely have discovered that the timing is way out - go dril some new mounting holes for the sensor screws or start filing the senso slots to get the sensor to the right place as my experience has been they are either close enough or a long way out.