How does one assess one's chances of getting accepted into a PhD program in philosophy? I'm about to complete my B.A. in philosophy, and I want to pursue a PhD in philosophy. I'm aware that it's a long, difficult, lonely process and that doesn't scare me off. But I can't help compare myself to the grad students who TA in my school's philosophy department. In particular, I lack the clarity of argument (especially verbal) and originality of thought that they regularly exhibit. I know these skills are developed, but I wonder if any part of them is innate. That is to say, it seems to me unlikely that everyone who desires to get a PhD and go into teaching is capable of doing so, just like not everyone's capable of being a professional basketball player, regardless of desire or training. Obviously you should go for your dreams, but it's important to be pragmatic and recognize your limitations. 

My question is this: what would it look like if someone wasn't cut out for a PhD program, and how would they know? I ask because I want to be sure (as much as is reasonable) that I'm capable of being accepted to and eventually completing a program before I invest a lot of time and money into the application process.
As everybody in the comments are saying, this is a very individualized thing. However, to get a good grasp of your standing relative to other students, I would check out thegradcafe.com.

Yeah, thegradcafe is an especially anxious and paranoid subset of grad school applicants, but you can see what kinds of applicants got accepted or rejected by checking the app board or skimming the forums. There's a dedicated Philosophy forum.