On the inside of your wheel, you will find markings similar to this:

6 ½ J x 15 H2 5/112 ET39

Let's see what each of these symbols mean.

6 ½
Inner width of wheel in inches (6 ½ x 25.4mm= 152.4mm).
Indicates the shape of the wheel on the place where the tyre bead sits on the wheel.
Indicates that the construction of the wheel is one-piece.
Nominal wheel diameter in inches (15 x 25.4mm= 381mm).
Type of wheels flange. Wheels flange is a bulge on the wheel in the place where the tyre bead lies on the wheel. It prevents the tyre from falling inside of the wheel.
First number is the number of bolts (in our case 5). The number 112 indicates the so-called PCD (Pitch Circle Diameter) which is the diameter of an imaginary circle drawn through the center hole of the screws on which the wheel is mounted.
Measuring PCD
ET is the offset, that is the distance between the centerline of the wheel and the plane of the hub-mounting surface of the wheel. The offset is measured in millimetres and can be positive or negative. In our case it is positive: 39 mm.

Related articles: