If you need find a products what not applied to website you can execute this query and it return a list of all product ids whta not applied to any website. Can also improve this query for retrieve any need fileds besides entity_id like a sku etc, or can set specific website id for check it.

Code:

../etc/congif.xml

Initialize event in front section

    <frontend>
      
    ...

    <events>
        <controller_action_layout_load_before>
            <observers>
                <VENDOR_EXTENSION_NAME_model_observer>
                    <type>singleton</type>
                    <class>EXTENSION_NAME/observer</class>
                    <method>setTemplate</method>
                </VENDOR_EXTENSION_NAME_model_observer>
            </observers>
        </controller_action_layout_load_before>
    </events>	

    ...
      
    </frontend>

../Model/Observer.php

Add observer for catch event

../layout/EXTENSION_NAME.xml

Setup in our layout blocks with custom markdown for csutomer gropus. We can use this for any conditions.

  <layout version="1.0.0">
      
      ...
      
  <CUSTOMER_GROUP_NOT_LOGGED_IN>
            <reference name="REFERENCE_BLOCK_NAME">
                <block type="BLOCK_TYPE" name="CURRENT_BLOCK_NAME" as="CURRENT_BLOCK_ALIAS" before="-" template="CURRENT_BLOCK_TEMPLATE"/>
            </reference>
        </CUSTOMER_GROUP_NOT_LOGGED_IN>
            
        <CUSTOMER_GROUP_Genearl>
            <reference name="REFERENCE_BLOCK_NAME">
                <block type="BLOCK_TYPE" name="CURRENT_BLOCK_NAME" as="CURRENT_BLOCK_ALIAS" before="-" template="CURRENT_BLOCK_TEMPLATE"/>
            </reference>
        </CUSTOMER_GROUP_Genearl>
            
        <CUSTOMER_GROUP_Wholesale>
            <reference name="REFERENCE_BLOCK_NAME">
                <block type="BLOCK_TYPE" name="CURRENT_BLOCK_NAME" as="CURRENT_BLOCK_ALIAS" before="-" template="CURRENT_BLOCK_TEMPLATE"/>
            </reference>
        </CUSTOMER_GROUP_Wholesale>
            
        <CUSTOMER_GROUP_Retail>
            <reference name="REFERENCE_BLOCK_NAME">
                <block type="BLOCK_TYPE" name="CURRENT_BLOCK_NAME" as="CURRENT_BLOCK_ALIAS" before="-" template="CURRENT_BLOCK_TEMPLATE"/>
            </reference>
        </CUSTOMER_GROUP_Retail>
      
      ...
      
</layout>