Wires aren't supposed to be installed any closer than 1-1/4 in. from a penetrable surface (the outside of the drywall). That means you shouldn't be drilling holes right next to the drywall. But it's not always easy to control where a flex bit goes. A Bumper Ball flexible drill bit guide installed on the end of your flex bit will help maintain the proper space between the bit and the outside of the wall cavity. You can buy a set of two at electrical suppliers or at licensedelectrician.com.
Cheaper multimeters can measure the voltage of mains, but we will suggest not to use the multimeter for that purpose as it might be difficult to change the fuse in a $2 multimeter. Other reason not to use is – if your fuse is blown away, probably it can not be replaced