Well, that very much depends on what validation you want. The easiest way to do this is to link input fields to script buttons so that the scripts get invoked when the user presses enter. You can then look up the corresponding data - material number or whatever - and issue error messages as appropriate.
One thing I plan to do, aside from just validating the material number, is to display the description. MB1A doesn't display the material description and that's a source of errors for us when users think they know a number and mis-remember it or mis-type it and so issue the wrong thing.