looking for a device or tester to know if car is electrified
EDIT -

While addressing the larger picture, I did fail to answer the specific question. The device you want is called a voltmeter. The most common version encountered nowadays is the Digital Voltmeter (DVM). To use it , you hold one lead in one hand, making contact with the exposed portion of the probe, then touch the probe of the other lead to whatever point on the vehicle you want to check. The resistance of the meter will limit any current flow through you to safe (and probably undetectable) levels.

In doing this, your body will act as an antenna, and produce a certain about of erratic reading, but a real voltage will produce a jump and a steady reading. 

END EDIT

Sorry, but your mechanic is simply wrong - at least in detail.

If you short any part of the wiring to the car body, you will not produce a hazard to someone touching the body. To do this would require two separate shorts: one at one potential to connect to the body, and another at another potential to connect to a dangling wire which contacts the ground. Without two separate connections at different voltages no current can flow and there is no hazard. Classic example - how do birds and squirrels sit and walk on power lines?

Even if the body panels are metal (and hybrids often aren't, in order to save weight), the enamel/paint will act as an excellent insulator, so simply touching the body will not allow current to flow. This condition of insulation does not apply, of course, to touching exposed metal such as trim and door handles. Nonetheless, just touching an exposed metal surface will have no effect unless some other part of you is touching a different metal surface which for some reason is connected to a different exposed wire from the first.

Murphy's Law applies here, of course, so there may be weird situations which will bite you, but they tend indeed to be weird.

Much more likely, for exposed conductors, is simply burning out electronics or literally shorting out the battery with an attendant pyrotechnic display and possible fire. Not to mention killing the battery. 

So, rats in the wiring is indeed something to worry about, but "electrocuting anyone who then comes along" is not really on the list.