Validation of Inputfields from Step to step

hi,

we have an GAF with a lot of steps. On the steps are only generic UIBBS(Feeder). All UIBB will be controlled by APPC  and the data sharing is implement by singleton classes.

From step to step we want to validate the currently displayed data. for example from step 1 to step 2 only the mandatory fields of step 1.

Now we are not sure, where we can check the input fields.

Which possibilities are there ?

1. First of all we can check the fields directly in the Feeder.
2. .....is there an other possibility in the APPC ????. How can i make sure, that i only check the data which shown in the screen?
3. ...........
4. .......

thanks