It is unthinkable that the vi heathenlovers will have a feature which we enlightened (X)Emacs users lack. whitespace-mode is what you need!

You might need to load the library (either add "(load-library "whitespace-mode")" to your .emacs, or if you just want it once, say "M-x load-library RET whitespace-mode RET"). Then, to activate it, hit "M-x whitespace-mode RET" and watch your spaces and tabs light up in different colours.

Note to the unenlightened: you never really type in these long sequences at (X)Emacs: you use TAB or SPACE to get completion.