Well the first glaring difference is the body of the truck. The 1st Gen has the old squared off body that they came out with in the early 70s. Underneath, the transmission is the 3-speed TF727 auto, the 4-speed A618 auto, or the Getrag 5-speed bolt action.
Under the hood, the main difference visually is the intake hose and injection pump. The 1st Gen intake picks up air from the front right of the engine bay right behind the head light. Later models used a pig tail intake hose that picked up air from the back right corner near the fire wall. The VE injection pump is distinguishable by being shorter in length, all the fuel lines come out the back (towards the fire wall) in a circular pattern, and it has a circular cast housing on the very top of the housing. The P-pump has all the fuel lines coming out the top in a line. The last clue to it being a 1st gen is if there is still a valve cover shroud on the top. The 1st Gen shroud says 160hp 400 ft-lbs stamped on it.
AS for electronics, the 1st Gen '89-'93 truck's only had an electronic ABS sensor and the radio controls. No other electronics; the cruise control had an electronic switch but was vaccuum controlled but the drivetrain was completely mechanical. These are teh VE pump trucks. The 2nd Gen 12-valve trucks which lasted from '94 to '98 were also manual injection but did have a few other electronics in the body for things like ABS. Thes are the P-pump trucks. The 2nd Gen 24-valve trucks went from '98.5 - '02 and used the electronically controlled VP44 pump. These trucks are fully electronic. And of course all the 3rd Gen from '03 on have been using the CP3 common rail system which has more electronics than you can shake a stick at.