I suppose that you use a button with text to navigato to the second Activity, so you can try
solo.clickOnText("second activity");
solo.waitForActivity(YOUR_ACTIVITY_CLASS);
solo.goBack(); // back to the first activity
ProgressBar pb = (ProgressBar)solo.getCurrentActivity().findViewById(YOUR_PB_ID);
int progress = pb.getProgress(); // here you have the progressbar value
See more here: http://developer.android.com/reference/android/widget/ProgressBar.html#getProgress()