The primary source of information is the Evaluate tab in your scenario.
Evaluate tab
By hovering over the nodes, you can see how customers moved through the flow, how many were processed successfully, if any issues occurred, and how many exited the flow due to an error. These metrics are available both as cumulative totals and for the most recent trigger run.
For condition nodes, hovering over the node shows how many customers matched the condition and how many didn't.
Important: The Evaluate tab shows where customers dropped off, but not necessarily the reason. For an explanation of what happens at each node, see "What happens when customers don’t match a condition or fail a scenario node?".
Best practices
Track customers at specific drop-off points
To obtain precise information about customers who drop off the scenario through unconnected outputs, you can track a custom event using an Add event node or track a value in a customer attribute using a Set attribute node. You can use these nodes as checkpoints wherever appropriate in your flow.
For instructions, see the Work with operators documentation.
Track Jinja aborts
If an action uses dynamic Jinja logic, the {% abort %} command can stop the action for customers who meet specific conditions. The customer doesn't proceed to subsequent nodes.
You can include a custom message to record the reason:
{% abort: "Custom message" %}The abort is visible in the scenario's test or evaluation information, and the custom message is stored with the aborted campaign event. For more information, see Stop execution for specific customers.