The Macintosh Voltage Meter displays the 3 main voltages: 12v, 5v & -12v (if available).
The device plugs directly into the external floppy port (DB19)

Compatible with:
Macintosh 128k
Macintosh 512k
Macintosh 512Ke
Macintosh Plus
Macintosh SE
Macintosh SE/30 (no negative voltage)
Macintosh Portable (no negative voltage)
Macintosh IICii
Macintosh Classic

I made these tester myself for my own  and now selling the surplus.