Thanks for your question.

As an aside, if you go into my index, you can find a few posts I have made in regards to the MCAT as well. From my experience of taking it twice, I can say that I definitely did not go into the first exam having walked the right path the whole way leading up to it. I must have cut a few corners somewhere because I did very poorly.

It is really hard to put the experience of the MCAT into words because it is not only a test of knowledge but a test of mettle. You really have to experience once to really know if you did it right. 

But there are ways.

One way to know if you have been studying and preparing well is to take a class. There are many programs like Kaplan that offer preparation courses. You get to go through questions and answers together with a group and with a tutor, someone who has done the MCAT before. It can be really great feedback and yield excellent results.

The second option that you can look into is to do practice exams. Many of them. There are many preparation books on the market and they offer a few exams that they have created, tailored to imitate the exam questions you will see. Usually they give about two to four practice exams. I would however recommend going to the source: the AAMC. 

They have a testing centre online where you can purchase old exams for a limited time and practice them. These are decommissioned questions but actual questions from past exams so it really helps to show you the level of questioning that you can expect. Not only that, they generally have a large set of old exams you can practice on. This, I feel is well worth your money, not only because it is a bank of high quality questions to practice on but it allows you to really get comfortable with the stamina aspect of the exam. 

Hope that answered your question. Cheers.