If your ads display on irrelevant searches, then either you get clicks and waste money (as the searchers are very unlikely to convert), or you don’t get clicks and CTR drops (which affects Quality Score and therefore your CPC).
So how do you stop this? Part of the answer is choosing keywords carefully – be careful with general terms and use modified broad-match where appropriate – but for the best performance possible you need negative keywords to target your traffic properly.
1. Search Query Reports
The most obvious way, but also the most important. Look at your search query report and see what terms your ad is actually displaying for.