How do I avoid an overbooked GCE zone?

Apparently if too many people use a zone, you won't be able to start instances back up.

Starting VM instance Error: The zone does not have enough resources available to fulfill the request

As I'll need to move the instance to a new zone, how do I know which zones are at high capacity so that I may avoid those?

I'm thinking of moving it from us-east1-b to us-west1-b, as the latter has been busy for over 3 hours

The information you are looking for is not publicly-available. However, if you know what your needs will be in the longer-term, you can utilize committed use discounts, which will guarantee capacity for you in your zone of choice.

I'll also mention that due to the highly transient nature of many public cloud deployments, instances of resource contention may be very brief, lasting for only minutes perhaps. A corollary of this is that even if you could know which zone is least loaded at one moment, that information has a useful half-life measured in seconds.