Measure it would be the obvious answer, but I'm going to guess it's buried out of sight.

Do you know if it's got any gearing between it and motor?

If it's all totally unknown, I'd set the steps/mm to 10 or 100. I'd probably try 10, and if you don't get much movement jogging, up it to 100, or maybe even 1000. You want to be able to jog at a reasonable speed. Too low a setting and it'll hardly move, too high and it'll be hard to control.

Then jog over a reasonable distance that you can measure accurately.
Then divide the commanded movement by the actual movement, and finally multiply the steps/mm by that figure.

So say you use 100 steps/mm. You jog 20mm, yet the machine moves 35mm. 20/35 = 0.571439 Now multiply that by 100, and your steps/mm should be 57.1439.