Dig out some of the clover and look for nodules on the roots. If you see nodules, the necessary bacteria are present in the soil.

Most of the clover(red, white, ladino) that I've bought in the last 10 years is pre-inoculated, and pelletized inside a thin coating of lime.
However, if you've purchased non-inoculated seed, the cost of a bag of appropriate inoculant(probably not more than $3) is a minor investment, and may pay big dividends, even if there is already established clover in the area.

The innoculant is cheap insurance. Dampen the seed and mix with the innoculant and plant immediately.

Plant in spring. Clover is quite cold-hardy. It also prefers acid soil to alkaline soil.

Sue