Setting up Email Alert if the Speed drops from ISP

The only way to verify the real bandwidth is some reliable speed test but doing so you will use that bandwith... it isn't a good choice.

Moreover if your throughput is 5 Mbps at a certain moment, you wouldn't know if it is for some ISP problems or because your users aren't using the full bandwith.

The best I could tell you is that you could measure the outer interface usage via SNMP and in this way you could have a precise idea or your combinated "ISP - users" behaviour.