A great place to start is with your policy documents. If your car insurance is up for renewal, take a look at your renewal letter as your discount should be outlined there. Alternatively, if you’ve decided to switch insurers, you may find your no claims discount is on your cancellation letter. Some insurers simply write letters stating that you have a bonus, as well.
But if you’d like to know before deciding whether to renew or switch, then just call up your insurance provider and they’ll be able to tell you how much bonus you’ve accrued. If you’ve decided to switch, you’ll need to supply proof of your no claims bonus, so your current provider can either note this in a formal letter or contact your new insurer directly. Each insurer handles the process differently.
If you don’t have a discount but are certain you’ve not made a claim, then contact your insurance provider to discuss why this might be.