It can be very easy or a bit difficult, cheap or expensive depending on how much data you want, how neat of an installation you want, ease of access to a suitable point to connect to the wiring and ease of routing wire to where you want the gauges.
If you are really into this and want to monitor amps in and out and more then consider getting a TriMetric Battery System monitor: http://www.bogartengineering.com/products/TriMetric
Or you could go low cost and not-so-neat like I did: http://www.classbforum.com/phpBB2/vi...hp?f=12&t=2257
I can monitor volts and amps in and out and more - I'd recommend buying a bit better stuff than I did if it is important to you. I rarely look at my monitors because they aren't mounted where I can easily see them. They are fun to have though when I am testing things or want to report what a my furnace fan draws for example. I have an inverter that displays volts (digital) so I turn it on to see the volts and that really is enough info to know the basics of what is going on. My solar controller also shows volts and has a light that indicates charging.
There is tons of stuff on eBay for display volts and amps if you don't want to go for the full featured TriMetric.