Check Field in the transaktion CRMD_ORDER

Hello Experts,

I need to check various fields to specific issues in the transaction CRMD_ORDER .

For example: first it should be checked if the entered item number exists.

I've recently started to develop and for me it´s verry hard to find the appropriate function blocks with the required functionality and to modifikate so that they can solve my problem

Can anyone give some tips for the correct function block.

For help I would be very grateful.

Thanks in Advance