How do I tell what width chain my used chainring needs? I bought a used crankset from my local bike collective. It has a single 44-tooth chainring and I intend to use it for a single speed conversion with a new cog and chain, but I need to figure out what dimensions of those latter components to buy. 

Most advice I see online says that single speed and internal gear hubs use 1/8" chain, while derailleur bikes use 3/32" chain. Great! Doesn't help me here: I don't know what kind of bike this crankset came from. It's only got one chainring, but that doesn't mean it couldn't have come from a bike with a rear derailleur and 6 speeds (or what have you). 

So, I resort to measuring. Is it as simple as 'if the width appears to be 3/32", that's what I need'? Or are there subtleties in the dimensions of the chain and the chainring that I need to look out for?
If it's 1/8" then a 3/32" chain will have to be jammed on to mesh, or not fit at all. The simple thing to do if possible is take a chain from a derailleur bike or even a scrap and physically try it. 

1/8" chains and cogs/freewheels tend to go longer and on a singlespeed application, there is no downside (other than weight) to running them alongside a 3/32" ring, so you could also just get 1/8" for the rest of it regardless of what the ring is and call it good.