“Isn’t it a big risk committing to marriage after only knowing the person for a few weeks? “
Yes
“is there any way of ensuring that won’t happen?”
No.
That having been said – all of life is a risk. You do your hishtadlus the best you can (due diligence), and try to make informed choices before you act. Some things are felt in the gut, not in the mind. I believe STRONGLY in getting to know someone more than just a few weeks before taking such a step as getting married. You would take more time picking out a new car than many people take picking a spouse. Soem people DO know right away, and they have wonderful, happy marriages. Others go out for months (and in some cases, YEARS) and the marriage is l’chatchilah not a good one.
Moreover, people can change with time. They grow, they are affected by life experiences in both a positive or negative way. They rarely remain exactly as they were when they were first dating.
We have to be able to adapt and grow with each other. There are NO guarantees of a happy marriage. But there is ONE guarantee in a Jewish home. Without a proper Torah foundation, and without the willingness to grow together, be flexible and willing to compromise, to be moichel on one’s wishes and kovod from time to time, a marriage has a very poor shot of lasting.