Support for this will become easier in platform version 8.2, allowing for capturing values post execution of the rule, but before the assertions. In earlier versions, an alternate to refactoring the rule (for better testability), would be best to have a wrapper activity that calls Activity A and then to capture the Parameter page and return it as an output argument to the wrapper activity or a named clipboard page that can be referenced from the PegaUnit test.
Properties on parameter page can be asserted directly. If activity B or activity A is setting some out parameter, it must be available on the parameter page. In any property assertion card, click on add properties link to open the property addition view. Click on the Add row link on the right side card to add a new empty row in the property assertion card. In the newly added row, one can enter Param.<Param Name> on left side, select the appropriate comparator and provide the value against which it has to be asserted.
We shall try to make this more explicit or easy to assert on parameters in upcoming releases.