All property acquired during the marriage is presumed to be community property.
The burden is on you to prove that it is your separate property.
Separate property is inherited money or gifts or recovery for personal injuries sustained during the marriage (except for loss of wages). In personal injury lawsuits, the settlement agreement becomes vital and must be disclosed if you are claiming that the money is your separate property.
However, if you add your separate property to a joint bank account then you have co-mingled it and it is presumed that you intended for it to be community property. It is presumed that you intended to "gift" this money to the community.
In order to prove separate property, you must "trace" the money from inception to current date. This can be difficult to do. It is also expensive and can require expert testimony to establish which portion of an account (or other property) is still separate versus community property.
Talk to a family law attorney in your county for guidance on this technical and expensive issue.