Unfortunately there are no alerts available for this limit. In a previous post about whether this could be verified, I had advised checking with the in-app chat support as well since they have more insight about it per Atlas account.
To my knowledge, the only way currently is to manually approximate using calculations from the Network chart for shared tier clusters. You could change the zoom to 1 week and adjust the granularity. I believe the limit is for all nodes (i.e. You need to add up the network usage for all 3 and then if that total value exceeds the limit, you will be throttled). Hope this makes sense.