Dimension fieldnum in forms

Hello.

I want to call the validate method of Dimension[x] form the form's datasource in one of the form's methods.

(eg: VendTrans_ds.object(fieldnum(VendTrans, Dimension[2])).validate();)

I know there is a function which can distinguish the number of the Dimensions/array field, but I cannot find it. (like when you use ranges on different dimension fields)

So how can I set the number of the array Dimension in my example?

Thanks a lot,

NaM