Clearly the definition of tiers varies over time. I think that the
tools needed to "measure" the current tier-ness of providers are:
a) a full bgp table (not just a full route table... if net FOO appears
behind AS3561, AS1239 and AS1 then you want all three entries).
b) a suitably large group of companies whose net access depends on how
you extract a route table from the full bgp table
c) a stopwatch
d) a phone
The procedure is basically to eliminate an AS from getting in the
route table and timing how fast the phone rings. Then you just need a
scale, something like this
0 - 2 minutes: Tier 1
2 - 10 minutes: Tier 2
etc.
In some cases you can replace the stopwatch with a calendar.
You can flesh out this methodology with extras like frequency of calls,
duration of calls, language used in the calls, whether you are threatened
or not, whether your family is threatened, etc.
This technique has been used by some service providers
by way of their peering strategy.
-mark