So, even testing the health of the ACC port +V line can be dangerous. But since that line is always connected to +V of the power supply or internal battery, you may use the following safe trick.
Power off the FT817. Unplug the internal battery. Remove any external power supply but leave the power cord connected to the radio. Get hold of a 1.5V cell and find out how to use it to power the FT817 (button cells can be held in place with a clothes peg).
The RTX will obviously not power up, but an accidental short circuit on the ACC line will not damage immediately the internal 10 ohm resistor (P = V^2/R = (1.5)^2/10 = 0,225W, about 1/4W instead of 14W at 12V!).
Extract +V and GND lines from the ACC socket with two insulated wires and measure voltage across them with a voltmeter. If you read your battery's voltage out of the ACC port, then everything should be fine.
You may also try to guess the internal resistor value, in case you have doubts it was damaged before. How? Connect a 10 ohm resistor across your loose wires and measure the voltage: it should be half of the battery reading. Or use Ohm's law to do the reverse engineering.
This procedure only tests the health of the +V ACC port line, not the whole ACC socket health.