Fairly often when I am trying to figure out the right brewing
parameters for a tea, I get a cup that is too much of something, but I
can't always tell whether it's too strong or bitter.

Is there some way I can learn how to tell the difference?

I think I can recognize when it's gotten very bitter, because of the
"bite". But when it's only slightly bitter, I have difficult
distinguishing that from just too strong.

Also, some of the strong blacks, like Assams, have a taste that is (to
me) a lot like bitter even when brewed for very short times.

Is it true that bitterness only occurs from steeping too long? I would
think that no tea would be bitter in a 30 second steep no matter how
much leaf is used. Is that right?

What I have done so far is try two more cups -- one at double the time
and one at half the time. That usually makes it clear. But I'd like to
learn to tell better just from tasting.