To make sure you haven't missed any clinical trials in your database searching, you can also "Hand Search" or browse the Tables of Contents of a journal or conference proceeding over a certain number of years.
Dickersin, Scherer, & Lefebvre (1994) explain that this can be a useful addition to your database searching, to allow you to identify trials that have not been included indexed by databases. Even when clinical trials are included in the databases, they may not include relevant search terms in the abstract, title and keywords that allow them to be identified easily as trials.
Dickersin, K., Scherer, R., & Lefebvre, C. (1994). Identifying relevant studies for systematic reviews. BMJ, 309(6964), 1286-1291. doi: https://doi.org/10.1136/bmj.309.6964.1286
Identifying the most relevant journals to hand search is an important part of the approach. Both Scopus and Web of Science allow you to perform a search and then identify the journals that hold the most number of results.
Run your search in Scopus
Select Analyze Search results at the top of the Results list
Scroll down and select the box that is displaying "Documents per year per source"
This will display the list of sources by number of results - this will identify the Journals you should be browsing
Web of Science
Run your search in Web of Science
Select Analyze Results at the top right of the results list
On the left menu, select Sources
The top level sources in the results will be displayed visually at the top of the page, scroll down to see a list.