What should I do to find out if the money was stolen or if I imagined the amount was larger?

You are not permitted to accuse anyone without proof. You can seek help from security agencies to search for the lost money, while being careful not to go to fortune-tellers. There is no evidence specifying particular verses for the return of stolen items, but it is legislated for you to recite the Quran and ask Allah through it to return what you have lost, and to seek assistance through prayer.