I have always been taught that in every trial I may encounter I am meant to learn something! Is this true?

I think it's true. It seems the trials also get harder and harder the farther you go or maybe that's just my perception.

How do we know if we are passing the trials, when they just seem to keep getting more difficult?

We might consider that the opposite may be true? That is, trials are not meant to teach us something but encountered to unlearn something. When trials are at our door step we are closer to that which we seek. Trials help us to simplify our belief until we have a childlike relationship with our universe.

Trials vary....from small to "your worst fears" realized.....
I feel that trials are meant to twist/mold you into who or where you should be going, and that hardest trials are the ones that define your life. I agree with LightningStrikesTwice, most trials make you question or unlearn unhealthy attitudes you may feel or have, and unless you quit the trial does not end. For some people trials never end because they never change. For some very special people trials reshape their life, but they ususally hit the lowest of lows before this happens. It is very true; "what does not kill us only makes us stronger?"....When you feel weak tell yourself...."I will not lose!"....and go forward. Churchill said it best...."If you are going through Hell....Keep going!"