OK, not quite what you are asking for, but maybe it will get you going.
To help a customer I spent ages thinking about how to test proximity sensors, encoders and flowmeters before they were shipped to site. The bits you need are a 24V supply, inputs that have the same specification as a PLC, LEDs that show when the signal is present. After a while I slapped myself on the back of the head, the obvious answer was a cheap PLC. Even cheaper, I had a old PLC sat on the shelf, it was 240V supply, but had a 24V d.c. supply output, so all I had to do was connect in a mains lead and I had the perfect tester. Just to make thing more interesting I programmed in a simple sequence so that as it received pulses it switched the outputs in sequence to confirm that the input was being received.
Not quiet handheld but what better way to emulate a PLC input than to use a PLC input.
Bryan