Start by checking the search results to determine if you need to improve or change your search. You will probably need to adjust your search several times before you are satisfied.
In most scientific databases, it is possible to choose so-called advanced search, which allows you to fine tune your search. For example, you can choose which fields you want your search terms to be found in, such as titles, abstract or keywords.
Do you get too few hits?
Do you get too many irrelevant hits?
Keep in mind that that if you get hits that are not relevant, this does not mean something is wrong with your search. This is completely in line with doing a systematic search. The important thing is that the search also captures the hits that are relevant and that the total number of hits is manageable. One way to validate your search is to check if relevant publications you already know of are included in the results list.
Read more about different techniques for imporving your search in the guide Searching and Writing.
A good guideline is that approximately 10 % of your search results should be relevant to your query. This means that you have a good balance between a broad and a specific search.
To test this on a database search, sort the hits by most recent publication date and go through the first 200 hits quickly. It is enough to look at the title (and if necessary, the abstract) to make a quick assessment of its relevance. Make a note of how many are relevant and calculate the percentage.