How to prove child support payments were sent by US Mail?

My ex-wife is holding onto the support payments (personal checks) I have sent her, and states to our children (age 7 & 12 )that she does not have any money for them because I have not sent her the past two months support payment. I have spoken to her on the phone recently prior to my conversation with our children and she did not or has not mention any of this to me. I've tries calling her, but she refuses to return my calls. I have checked with my bank to see in fact that she has not cashed these checks. We have a very hostile relationship, as she is very bitter and We are due to battle in court (again). She is unhappy about her spousal support being at $0 and the split of community property issues, which is not in her favor. I believe she is planning on trying to use this lie in her efforts to gain more money. What can I do to protect myself, I have no proof of sending these payments. What are my options for this, as she holds these checks.

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