Toggle between multiple purchase order

I am working on purchase order(PO).

I have performed some manipulations on the user defined field on this PO.

User defined should be opened for performaing calculations.

The problem is created when multiple forms are opened without user defined fields.

I am getting the unique id of PO but I can not relate the user defined fields form with the PO if multiple forms are opened.

scienerio is that

i have opened Purchase order form (unique ID F61)

again i opened purchase order form (unique ID F62)

Now i have opened UDF form of Purchase Order F61, (unique ID for UDForms is F63)

Now my active form is F61,

then tell me how can i find out wheather User Defined Form of PO F61 is opened or not?