On the mixer, there is a PFL(Pre Fader Listen) monitor switch. You're meant to use this to monitor the input levels with headphones. I suppose you could buy a monitor meter and hook it up to the headphone jack. You could then use the PFL switch to monitor different channels.
However, while that seemed like a good idea, I couldn't find any headphone monitoring meters. Which is quite surprising. You could build one with some components from Maplin or some other electronics store if you're capable.
The only thing similar I found was this rack-mount VU meter which would produce a very satisfying visual display, how beautiful would that be, hmm?.
It has a nice, stereo RCA input and output as well as input level control. I think this would work, but I would check the headphone output level, just to be sure it would satisfy a line input. If not, you may need to buy an additional level switching device, but I doubt that would be necessary.
If you do some shopping around and maybe phone some electronics stores, you may be able to find something smaller and cheaper.