Ok, first you need to determine if it is one of the so-called " / 2 " bikes. These are the 1955 to 1969 Boxer Twins. If it is not a " / 2 " you won't find out much about it here....
1. What do the shocks and the valve covers look like?
If the shocks are low and the valve covers look like the one in the picture below, chances are it is pre 1955 or possibly prewar... that is, made before World War II.
2. What kind of drive shaft does it have? This shaft is open, no enclosure. Slash Two's have an enclosed shaft drive.
3. Is the engine mostly enclosed in a smooth housing or is the starter exposed? Slash Two's have the starter enclosed on the top of the engine.
4. Early Slash Two's have Earls' Forks. Those "L'' shaped forks. But some later "US" Slash Twos have Telescopic forks so this is not a sure fire test of Slash Two-ness.
5. Don't forget we are talking about Twins here. If it is a single It is probably a R26 or R27. I hope to make a page on these in the future.
Though this is often unreliable (due to the interchangeabilty of these often damaged parts) it is a quick start. Then check the cylinders.
Next, the air filter housing.
Is it an R69? Check the front of the Engine:
If it has large headlight and horn switches chances are it is newer than 1960 ( likely a /2 ). These switches look like the ones on the /5 models.
If it needs clamp-on mirrors, then I suspect it to be pre-1960.
And lastly, check the rear end numbers. Numbers on the top of the final drive may indicate the age and model if original and was never setup for a sidecar rig.