Scotch in its purest form is entirely kosher. However, Single Malts can often be matured in Sherry casks. Sherry, as a form of Wine, is not kosher. Macallan Sherry Oak might be one to dodge, for instance.
The simple answer is, check the bottle. Distilleries such as Glenmorangie, Ardbeg, Auchentoshan, Bowmore, Glen Garioch, Glenrothes and Tomintoul are now producing kosher-certified Whisky. If you’re not sure, mourn the loss and move on.