To know for sure, you need to:
* Check your owner's manual: This is the most reliable source.
* Look at the lug nuts themselves: The size is often stamped on the nut.
* Inspect your wheels: The size might be stamped on the wheel itself.
* Measure the lug nuts: Use a caliper to measure the diameter and thread pitch. However, without knowing if it's metric or SAE this is difficult.
Be very careful when measuring and handling your lug nuts to avoid injury.