The truth is that you really can´t tell a -500 apart from a -300 while you sit in the cockpit.
Well, there are two things - one is the registration plate pinned to the instrument panel (our -500s had an I or a J as the third letter).
The other way to tell is to set flaps to 1 and shove the thrust lever forward on the ground, the -500 will get you the takeoff warning horn.
The -400 is actually a bit different, it has stronger engines (you can see the thrust limit on the FMC and also get slightly higher N1s at full power) and a slightly different airconditioning panel.
Cheers, Jan