In garden pea, violet flower colour (V) is dominant over white...

To determine the exact genotype of a plant showing a dominant phenotype (violet flowers, which could be $VV$ or $Vv$), a test cross is performed.

In a test cross, the individual with the unknown genotype is crossed with a homozygous recessive individual. Here, the homozygous recessive plant has white flowers ($vv$).

If the unknown plant is homozygous dominant ($VV$), all offspring will have violet flowers ($Vv$).

If the unknown plant is heterozygous ($Vv$), the offspring will show a $1:1$ ratio of violet ($Vv$) to white ($vv$) flowers.

Thus, crossing the plant with a white-flowered pea plant ($vv$) is the correct method.

Answer: Cross the plant with a white-flowered pea plant ($vv$)