Internet routing table "completeness" monitoring?

Wed Oct 3 04:43:09 UTC 2012

Has anyone put in place a method to identify if one their BGP peers 
suddenly withdraws X% of their prefixes?

e.g I should expect ~420k prefixes in a "complete"[1] routing table from 
a transit peer today.  If suddenly I'm only getting 390k prefixes I'd 
guess a major network was depeered or similiar.

If so how are people doing this? SNMP MIB, screen scrape?

[1] Varying levels of completeless apply.