I'm not sure what SIEM you are using but the best way to ensure this is to use the Archive the log when full option. Your SIEM should be able to process the archives and the current log. The other option is to get a count of events from known start time and end time. Then compare that number to what the SIEM has processed.
Also, it may help to filter the Security Log events for noise if you're not doing so already. Supercharger has a built-in Security Log filter with noise suppression. You can also create your own custom filter under Settings in Supercharger.