I opened one of my shocks to have a look-see. What exactly do they do to them when they rebuild them? There is hardly anything there. Also, how do you test them to see if they actually need a rebuild?

They normally wear at the pivot bushing though the housing, due to the seals failing to contain the hydraulic oil. When dry the bushing area wears very quickly.
There is also valves which allow the fluid to flow at reduced rates as well.

Fill your shock absorbers with the correct oil, and then bench test them for a few days, moving them up and down, and note if there is any oil leaking past the seal on the main pivot throught the Housing , would be a good starting place.

New gaskets are available from the fillingstation for the end cover plate.(from Memory)

Some are meant to be rebuilt and some are not! The rebuildable ones (kits from FS)have a screw that secures the pivot arm to the inside rocker arm that depresses the piston. By removing the screw, the unit can be dismantled and serviced. Others (the 4 that came with my '31 Canadian coach for example) do not have this screw attachment, and instead the pivot arm and rocker are a press fit, making them not serviceable (at least not easily). For mine, I took them to a machinist friend who had special gear to get the pivot arm and rocker apart to replace the seals, and then the valves etc, and to enable them to be pressed back together. He said he would not try it again as it was a very difficult task. I am not sure when Delco-Lovejoy changed from one type to the other.