How to use an OM extension to check specific items exist on the sales order

Summary:

I would like to give a warning message to the user when item A is entered on the order but item B does not exist on the order. For example enter item A on the order but item B does not exist on the order, give a warning message to the user on save. If item B is entered on the order and then item A is entered no warning message is necessary.

Content (required):

Version (include the version you are using, if applicable):

21C

Code Snippet (add any code snippets that support your topic, if applicable):