Battery indication on extremely simple 9V circuit
If you wire a led (say a 3.2V nominal blue led) to a 9v battery positive, and a wire to the 9v battery ground, lick your finger and touch the negative wire and the led cathode, it will act like a 1000 ohm resistor lighting up the led enough for you to know it's working. But to make it even easier, just like the battery. It normally wont kill you. I do it all the time to test 9V batteries. Not kidding.

That said let's avoid the unwise setups. Wire an led with a 1k ohm resistor and a push button to the battery. When you want to test the battery just press the button. No skin inbetween. You can put it anywhere between the two circles. And it wont affect the slugs deterrence as it will only draw current when the button is pressed.