Have a single shot short barrel weapon of unknown caliber. Was told it was handmade for either 38 Special +P or 357 Magnum. How safe is it to shoot 357 Magnum if made for 38 Special+P ? Maybe handload progressive 357 Magnum loads for test fire ? Or just shoot light loads ? I am setup to hand load 357 Magnum, but not 38 Special. Any thoughts ?
Both .38 Special and .357 Magnum are calibers specified by SAAMI. If you have a gun with no indication of its caliber and no means of verifying its compliance with SAAMI specifications then don't expect it to safely fire SAAMI-spec cartridges.
If a SAAMI cartridge appears to fit it and you want to work up a load then realize that you are engaging in proof testing of the gun, and you should learn and follow best practices for that to avoid unintended destruction of life, limb, or property.
If you are saying that you are sure it is designed for at least .38 Special +P then, as explained here, it can fire those. (If you aren't comfortable adjusting your loading dies and press from .357 Magnum to .38 Special length then it may not be prudent to play with down-loading .357 Magnum cartridges to try to match .38 Special pressures.)
I'm no gunsmith, but personally I wouldn't trust an handmade firearm built by an unknown person at all. How do you know that the maker was competent in firearm design, metallurgy, machining, and all the other things that must be well-understood in order to make a safe firearm?
You could try proofing the gun by shooting .357 Magnum ammunition in it from behind a barrier, pulling a string tied to the trigger with the gun in a fixture. But even if you were able to do so without the gun exploding, that would be no guarantee that it's safe; there could be invisible microscopic cracks propagating through the metal. It still might blow up the next time you shoot it.
You should probably consult a local gunsmith with a good reputation rather than taking the advice of strangers, but I'd be amazed if the gunsmith said it was safe to use for anything but very light loads.