Question of the day: So my question is, how is the poor author supposed to determine if she had a poor agent or a poor manuscript? Somehow I can't see a poor agent admitting that they hadn't really tried
Ooh, toughie. Well, if your agent has lost interest or you suspect your agent has lost interest, I'd ask to see the list of where he/she has submitted. You're entitled to this information. Some agents openly share it (mine does) as the process goes along, others are more cagey. But, if you're considering moving on to someone new, it's imperative that you know where your ms has been, and thus, your agent should be more than willing to let you know who has read it. If the list looks long and relatively complete, you might still want to poke around on Publishers Marketplace to ensure that the right editors read it, and if so, well, then, it's best to start writing something new. If it's short and pathetic, however, this might be your opening to seek another agent with the same ms: you'll have to share this list with the new potential agent, and it would be up to him/her to assess.
As to whether or not the problem is that the ms itself sucks? Well, this is something we've chatted about a bit before, and the problem with getting an objective feel for it is that we often aren't capable of this until long after we've stepped back from the ms. In my own case, it wasn't until I'd written a much, much, much better book that I realized that my first (unsold) effort seriously blew. But given that you might not want to take this route, I'd recommend getting it into the hands of objective readers: find a critique group - maybe online, maybe at your local college, maybe via a local writing workshop - whom you trust and see what they have to say. Listen with open ears. If you truly want to be sure that the ms is the very best it can be, you have to accept some constructive criticism.
Anyone else have a good method for determining when your work stinks? Or for determining if it's actually your agent, not you?