How can I prove thatan executor has taken money froman estate?

From the facts presented, it appears that there may have been a breach of "fiduciary duty" by the executor.  Such a duty is the duty implied when someone is put in a position of trust to benefit other, such as that between an executor and beneficiary.  Accordingly, a fiduciary must at all times exercise good faith and put their interests after to the interests of the beneficiaries and/or estate.  Additionally, the executor must follow the law as it pertains to the distribution and handling of estate assets.  The failure of a fiduciary to perform their duties gives rise to a claim.   

Based on the fact that they don't want to a credit report and, I assume, any receipts, account statements, or the like there may well be either fraud, negligence, and/or similar misconduct at play.  Therefore, you can contact the probate court in question and/or directly consult with a probate attorney on all of this.  You can challenge the executor in their fiduciary role and have them removed for breach of their fiduciary duty.  Additionally, you will then need to have someone else appointed who can challenge any transactions/transfers that may have been made which were not in the best interests of the estate and get the money refunded. You will also need to request for an accounting of the estate.  If the executor is bonded (insured) you may also be able to go after insurance money to recoup any losses. Again, a probate lawyer can best guide you in this.