IW41 and IW44 validation

Hi people! I need some help.

I'll need check the field PERNR and begin date AFRU-ISDD, hour AFRU-ISDZ and end date AFRU-IEDD, end hour AFRU-IEDZ. The idea is make a validation, to prevent user to save a new register period and pernr in a ordem that already had it.

This program is already using a field exit in field PERNR. I thinking to do this new validate in the same, but i don't know how I can get the values of Begin Date, hour and End Date, Hour...how i can get this?

Thanks a lot!!