More on point for this thread, I always have new vendors bring in fiber
maps and show me their paths. Images of the intended path specified on
the map are part of the contract, including verbage regarding failover
paths. Once I know where their fiber is, I can look for another vendor
that takes a different path.
This often won't get you the most cost-effective connections,
and sometimes it'll be bad for performance as well,
and doesn't always take advantage of available technology.
For instance, if Carrier 1 and Carrier 2 both use the same route for
their primary connection,
and you buy from Carrier 1 because they're 5% cheaper,
you may find that Carrier 2's second-best route is a lot more
expensive that Carrier 1's.
If you're buying from two carriers to get equipment diversity as well
as route diversity, you've lost.
Another kind of problem I've run into in the past - here in
California, to get from SF to LA,
you can either go down the coast or down the Central Valley, depending
on which railroads or highways you like. But there's another route
that takes a railroad connection from
SLO (middle of the coast) to Bakersfield (south/middle of the valley),
and if your primary connection uses that route, the options for
diverse routes go through Salt Lake City or Denver. Given the
history of what fiber got built when, you'll find that
for some speeds many of the carriers use that crossover route, while
for lower speeds
there's a lot more choice.
From a technology standpoint, a lot of carriers are starting to use
intelligent optical switches
that give them automated provisioning, automatic reroutes, etc., so
while they can show you
where their cable routes are, and where the most likely provisioning
and reroutes go,
in general you can't get a precise guaranteed route, because that's
not what the switches do.
What I find hard to combat are M&A changing operations over time,
In general, it's hard for one carrier to keep track of diversity
(though some can),
and much much harder for two carriers to keep diverse from each other.
And the tracking problems scale differently for large connections,
where you may build custom access rings, than for small connections where
most providers are reselling telco last-mile copper.
There's also the problem of diversity philosophy - it's not uncommon
for large East-Coast
companies to view equipment diversity as the critical problem, and
concentrate their
switches into a smaller number of larger sites where they can do
cost-effective sparing,
and have their fiber spread out across many different physical routes,
not remembering that customers in the West Coast expect that
buildings just fall down sometimes, so they care about building diversity,
and geographical and demographic considerations mean that there are only
a few good routes across the Rockies and along the coasts.
(Of course, sometimes this means that the West Coast customers buy from multiple
carriers to get building diversity and _still_ get caught when a telco
DACS fails 
....