You've explained how important is an agent's enthusiasm for a project. Other than what she says, how can a writer gauge an agent's enthusiasm? For example, do you respond more quickly to full ms's about which you're more enthusiastic? If Agent A takes 3.5 weeks to offer me a contract and Agent B takes 12 weeks, it seems logical to presume--the agents' status being equal--that Agent A is more enthusiastic about my work.
no. It could mean that A can run her own contract offers and B has to get approval. It can mean that B ran out of ink or is getting her contract updated, or she's busy swilling gin with Miss Snark.
It's impossible to give you any sort of concrete index by which to measure enthusiasm. It's like trying to measure love. You can tell which projects have captured my enthusiasm because they are the ones I yap about most to colleagues and friends. They tend to be the ones that generate the most email babble back and forth to authors. Of course, there's no way you can know that cause all you see is the email I send. You have no way to know if it's more/less than anyone else.
I do suggest though, that like love, you can sense when the thrill is gone. You have to be careful not to become an insane psycho clingy love monkey ("do you love me..do you really love me?? Prove it!!") but pay attention to your antenna which are pretty good barometers of love, and enthusiasm.