How can I be sure that my ex-spouse will maintain the life insurance policy that guarantees my alimony?

You could ask for periodic proof that the policy is maintained, such as providing you with an annual copy of the policy that shows that payment has been received for the year.

You could also ask for a provision in the policy that the insurance company must notify you if your ex-husband should stop making the insurance policy payments. This is very common do to the financial risks at stake.