technically, the "vmq_bridge_dropped_msgs" counter would show whether the bridge has gone over the configured buffer ("max_outgoing_buffered_messages") for its internal queue at some point. If the counter is 0, it hasn't.
Note that on the node where you started the bridge, you will have metrics for the incoming and outgoing messages, like "vmq_bridge_publish_out_0". This might give you more information on the number of messages flowing through.
Also note that a pull bridge creates an MQTT consumer at the remote node. If that bridge consumer is too slow for the sending broker, it (the broker) will drop messages. So you'll want to check for dropped messages logged in the remote sending broker's log files.
If you're overloading the bridge like that, then you have to re-architect/distribute this in a better way.
Hope this helps,