Yeah I usually ask if they would be willing and able to write a positive letter on my behalf.
One instructor did reply with a warning that he won't be able to write about much more than what appears on my transcript, and suggested that I ask somebody else if possible. On the other hand, he said he sympathizes with me if I have limited options due to schools requiring explicitly BCP letters, thus he would be happy to do it if I didn't have much of a choice.
Others I asked agreed without expressing any reservations.
In general, can I expect people to be that upfront if they have hesitations about writing me a good letter?