Internet routing table "completeness" monitoring?

[ML <ml () kenweb org>]

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.