I'm in the process of switching from the Legacy SFDC integration to the SFDC integration. How can I be sure that data syncing for Accounts, Contacts and Leads works in the new integration if both systems are active at the same time?
To test that the new integration is working properly, put a sync-filter condition on both the new and legacy integrations so that they act on different subsets of Accounts, Contacts and Leads.
If you prefer not to create additional fields on the object, use an existing field as the sync filter switch. By making a custom checkbox on the relevant object (Account, Contact, Lead) in SFDC called Sync Using New integration, you can configure the legacy integration to sync it, if the value is false and the new integration to sync, if the value is true.
In the legacy integration, this filter can be added in the User & Organization Sync settings as shown below.
In the new integration, add a similar filter by going to Admin Center > Platform > Salesforce > Data Sync.
With filters in place, test the new integration by checking a box on the records to be synced.
After you tested, remove the filter, and disable syncing through the legacy integration by changing Enable Updates to No.