Shoot a ruler and calculate. The ratio compares the size of the photographed object with its size on the film/sensor. If the size is the same, you have a 1:1 ratio or "life size".
In your case:
If the ruler shows 36 mm, you have a 1:1 ratio.
If the ruler shows 18 mm, you have a 2:1 ratio.
In other words, if the ruler shows X mm, divide 36 mm by X and you will get the ratio.