custoport mizing re

Hi experts,

i have customized a report in which i have added material group in selection screen.now i have wite validations for material group and material number.the condition is for a particular material number i have to check whether this material number is exists in material group or not .if materialnumber exists in materialgroup thn it should o/p else it should display a message in selection screen.can ny one help me in this issue.