Modules & VBA :: If Statement That Looks Up Values From One To Another Form

I have an issue that can be described as follows:

There is a textbox on a form and when I fill in its value, I would like it to lookup whether that value matches any of the values in another form and if it does present a message box. Both fields hold only numerical data, so no type mismatch issues.

I've tried the code following, but it only looks up the first value on the second form, not its entire dataset, which is problematic.

Code:
Private Sub Lot_AfterUpdate()
DoCmd.OpenForm ("Pot_Pot_ExtraLots")
If Me.Lot.Value = Forms!Pot_Pot_ExtraLots!ExtraPotLots.Value Then
MsgBox "There is a bag with extra sherds found during other analyses from this Lot! Lookup and combine results!"
DoCmd.Close acForm, "Pot_Pot_ExtraLots", acSaveNo
Else
DoCmd.Close acForm, "Pot_Pot_ExtraLots", acSaveNo
End If
End Sub