I am one of the coders of Grafx2. In this program, you'll generally want to use keyboard shortcuts for next/previous frame (pageup/pagedown, if I remember correctly) so you can compare frame 1 and 2, then 2 and 3, 3 and 4 etc. While doing it, the "adjust" tool (bottom left) lets you easily shift the frame until it connects nicely with the previous frame.

Other animation programs can have a function called "onion skinning", which lets you view several images at once : usually only "next", or "previous+next". Some can show more frames at once, sometimes with decreasing opacity (distant frames are barely visible). You should try for yourself, see if it's the feature you're looking for. I'm sorry I can't recommend a specific program, I tested all the free animation programs I could find, and never found this function as easy and powerful as I expected. IMO it's only useful at high resolution (96-pixel-high fighting characters) and during sketching, with thin outlines. In any other cases, there are so many overlaps of pixels from the two frames that you can't see which frame(s) the pixel belong to. And in these cases, the mechanisms of Grafx2 (switching back and forth between two frames) gives a much better idea of what you're doing.

The entire list is available from within the program : hit F1, click Help. The highlighted 'key' names can be clicked to modify the shortcuts, and you should really change them for familiar ones, rather than memorize grafx2's defaults.
But instead of browsing this long list, it's better to hit F1 for example while your mouse cursor is over a button of the toolbar, or while a window is open (palette, etc) : It will open the Help page about this button/function, and display just the relevant shortcuts.