You can check your mast and see that it is plumb. However, if you don't want to do that, you can place a level across the three eyes on the LNB and check it that way. To do that, record your current skew, then rotate the skew back to 90 and check if the level across the LNB shows level. If not, rotate the dish until you get level and record the number of degrees you are off from level. Post your zip code here and we'll give you the correct skew. Then you either add or subtract from that skew to arrive at what your skew should be after compensating for a mast that isn't plumb.