Using WMS Reflector
This is a great way to preview options in GeoServer without coding a long URL. The reflector will output PNG (the default), JPEG, PNG8, and GIF. Also, in cases where you don't want to use GeoWebCache, this is quite useful.
The URL passes a number of parameters to specify what output you want. Most of these will not be changed. Reflector uses default values for missing parameters. The only parameter you need to provide is the layers
parameter by default. Check out the GeoServer documentation for more information on these values. There's no need to rehash them here.
Note
For more information, take a look at http://docs.geoserver.org/stable/en/user/tutorials/wmsreflector.html.