measure the stud pattern. The Stud Pattern is the spacing between each of the wheel bolt/nut holes. This is known as P.C.D or 'Pitch Circle Diameter ' which is the measurement from the centre of each wheel bolt hole on the alloy wheel. if measurement was 100mm then the Stud Pattern or P.C.D would be either 4x100 or 5x100. Again if the measurement was 108mm then the Stud Pattern or P.C.D would be 4x108 or 5x108 and so on.

few examples
4x108 is ford, pugeot and old audis
4x100 is renault, vauxhall an volkswagen
5x114.3 is honda, hyundai, lexus, nissan, mazda
5x112 is audi, mercedes, vauxhall

just look up the rest.