While new to the "The Rejecter" as such, I thought I was familiar with rejection until I received a request for a full from a repudiable NY agency last July and have yet to hear a word. Could that be considered a rejection by omission rather than rejection by commission? What's the protocal regarding an e-mail to the agency and asking "What's up?"
Three months is the average wait for a partial or full. A good way to put it is, "I just wanted to contact you and make sure the manuscript was safely received." That's a good way of not stepping on any toes.
Let's say that you finally get that phone call from an agent offering representation. Are there any questions that you think writers should ask before making their decision to work with that agent? Is there any way to tell if that agent is the right fit for you?
It's very difficult. A good thing to do is ask them questions about where they would submit your novel and what they could do for its publicity. Try to get a feel for how excited they are about it and how committed they would be to the project.
If you have multiple books in multiple genres, try to get an agent who works in all of those genres. That's what I did when I was selecting between agents, and now my agent is helping me revise my second novel, which is sci-fi, for publication.