About using Box with Salesforce

Our company currently uses Box for Salesforce installed within our Salesforce environment. To further enhance integration with Box, we are considering adding new functionality to Salesforce. Specifically, we plan to develop a feature that automatically grants access permissions to the corresponding folder in Box when record-level access permissions are granted in Salesforce. In the standard Box for Salesforce specification, Box access permissions are granted when the user accesses the corresponding record in Salesforce. However, we want to be able to search and access Box content without accessing related records in Salesforce. To achieve this, we are considering using box.Toolkit, which is included in Box for Salesforce. Before proceeding, please allow us to confirm the following technical constraints and specifications regarding the use of box.Toolkit: We want to synchronize permissions to Box for Box users linked to Salesforce users. However, it is possible that Box users are not linked to Salesforce users. We want to exclude such users from permission synchronization. Is there a way to check whether Box users are linked to Salesforce users before the permission synchronization process (before creating the FRUP file)?