There are a few apps available that allow you to preview your film negative as a positive. A quicker way that I have been using for a long time to do it is to invert the colours on my smart phone. Im sure most smartphones can do this but i have an iPhone 6s and I know how to do this on iOS.
Settings, General, Accessibility, Display Accommodations. At the top of the list you will see ‘Invert Colours.’ Click on this and toggle ‘Classic Invert.
It inverts all the colours over the whole of the operating system – including the camera. When you then hold your camera up to a film negative, preferably on a light box you will see on screen the positive version.
…but thats not quick you say…!? Well no, thats not, but if you go again to your Accessibility settings, scroll right to the bottom and go to the Accessibility Shortcut tab you can set this up as a shortcut from the iPhones home button. Three clicks of the button from any screen and the screen will invert. Three clicks again and its back to normal.
Its no alternative for a scan, but helpful for quick reference.