You can turn it and listen to hear if it powers up, but ultimately you have to send it a signal to see if it displays anything.
I think if you have something sending it a sync signal, and the gnd input hooked up, you can then send 5V to each of the R,G,B lines and shoud see a solid red, green, and blue screen respectively.
But even then, it could be all out of convergence or screwed up.
There are plenty of cheapo jamma boards you could use, I use my contra board pretty much exclusively as a monitor tester, since it has decent crosshatch and color bar displays.