First up - confirm that its Roller Chain. There's a metal doughnut roller around each pin in this example:
If it doesn't look pretty close to this, then its not bicycle chain.
Then check if its a bicycle chain. The pitch will be exactly 1 inch / 25.4mm per pair of inner+outer links, with maybe up to 1-2% longer if its worn. This is the same as saying 0.5 inches between adjacent chain rollers/pins.
Any other chain pitch and you're looking at a motorbike chain.
Then move to measuring the width, between the inner plates.
- 1/8" (0.125", 3.18 mm) chain is used on most single-speed bicycles, and this would be my guess.
- 3/32" (0.094", 2.30 mm) chain is used on deraileur equipped bicycles.
Again if its not one of those two, you have motorbike chain, or possibly something different again.