I have recently completed my undergraduate degree, in mathematics. As part of the final year, we had to complete a "project", which was effectively a 40-page (ish) dissertation on a topic of our choosing. This was done with the aid of a supervisor, one of the researchers in the department, for the duration of the year.
During the undertaking of this project, I felt like I developed a good relationship with my supervisor (who I met once a week, and who provided a reference for me for PhD applications).
At times, particularly when knee deep in PhD application stuff, I started to get a little negative/worried about my long-term prospects. This seems to be a fairly common problem, from what I read here. On more than one occasion, I was seriously tempted to ask my supervisor whether he honestly thought I was any good, had any reasonable prospects doing maths/a PhD/research, whether I should do something else, etc.
I never actually ended up doing that, because I always decided that it was a bit of an unfair question, and that I my supervisor quite possibly wouldn't know anyway.
My question is: In a scenario like the above, is that ever a question that one could expect a supervisor to answer/answer well? Is it easy to tell, from a supervisor's point of view, what a students prospects are in that field?