Test to see if a movie clip is visible?? Instead of setting it

Im trying to see if a movie clip's visible property is true or false. If it's true then I want it to move the frame.. Instead my code is making the visible property equal to true on a swipe?? How can I only test and not set the property? It still swipes, just instantly makes it visible.

if(e.offsetX == -1)

                              {

                                        if(this.visible = true) {

                                        nextFrame();

                                        }

                              }

                              else if(e.offsetX == 1)

                              {

                                        if(this.visible = true) {

                                        prevFrame();

                                        }

                              }