I suppose there are a lot of different ways, depending on the nature of your signal and what standard you are trying to verify it to.
The simplest would be to connect an LED (preferably in series with a, say, 10k resistor) to the pin.
An old analogue oscilloscope might work well. I have one about 40 years old with I think 10 MHz bandwidth that I got for free. It’s pretty much fast enough for traditional AVR Arduino signals.
Or you could connect that pin to another pin configured as an input and each time you write the output pin wait a short time (1 us should be plenty, if you can afford that) and then read the other pin and see if the correct data is there.
Or you could use another Arduino-style board or a Raspberry Pi to make your own digital oscilloscope. There are a lot of ideas for that here https://www.instructables.com/id/PiScope-Raspberry-Pi-based-Oscilloscope/