I have some Zooz switches to install and I know their support is really great for this. But I also want to have a bit more comfort/understanding myself. Frankly, I think it’s mostly just overwhelming and I don’t want to get electrocuted, so breaker is off.
I have a “simple” box like this with all the needed wiring:
Left is on a 3way right is single pole. Even switching out the single pole, I need to bring in the neutral which I THINK means I can just tap into the bundle of white wires?
but for the two connected wires how do I actually identify which is for line and load? And be certain about it? Their manual says these can’t be switched I assume it risks breaking/frying the switch…
All the Black wires are bundled and that's most likely to be Hot/Line. A black wire in that bundle goes to each switch. Put that wire into the Zooz Line connection. What's left over is Load.
I'd suggest wiring up the Zooz and leave what you imagine is Load unconnected. If you have it wrong, the LEDs on the Zooz won't have power because you used the Load wire... so nothing will fry.
Obviously you should cap (wirenut) the unconnected wire during this test to prevent accidents.
The best my eyes can do with the pic is say that I think the left wirenut is hot/line.
For the switch on the left, you have one hot (black) and two Travelers (red and white).
Yeah the left I’ll have to double check the final wiring because it will be a 3way on zen76 and I know the wiring gets slightly more complicated for that.
For the right, indeed the bottom one goes to that bundle and the other goes straight to that conduit in the top left of the box. So it certainly seems you’re right (I wasn’t aware the bundle typically indicated the line but that makes sense understanding it a bit better…)
Interesting thought about only wiring up the one to “verify/test”.
Have also heard about using NCV but it doesn’t seem foolproof and is prone to false positives?
Electrical is… fun! Haha
I have another box to do with switches later, and it’s a 4 gang. Getting it all right is one thing. Getting it all right AND fitting all the wiring back in a more cramped box… eek! Haha
Overwhelmingly, getting everything to fit behind the larger switches is the biggest challenge.
The specific NonContact Voltage Tester I have works reliably, so far I'm sure technique has something to do with it. Mine has a sensitivity adjustment and I poke it into an outlet, adjust sensitivity to what I like and then when I use it inside an open wall box, it's been reliable. Angle of attack makes a difference too. Can't let any part of it get too close to any other wire, which is where sensitivity comes to play.
I replaced almost all of my downstairs outlets and wall switches/dimmers with Legrand Adorne line and they are bigger than standard also BUT that then forces me to put ZWave Modules into the box, and fight to get it all in there. But I'm not giving up the look of Adorne to make getting the box packed easily.
I think as @csteele explained (1) should be the line, and (2) is load, that is, if I traced the wired back to the bundle correctly. For (3) you can just add a new piece of jumper wire to the bundle for the neutral (you may need a larger wire nut).
Having & using a multimeter is always recommended, but a good NCV is handy too (especially for nailing down multi-way switch wiring). But some are much better than others...
Cheapo ones will start going off as soon as they get near the box, so that's not useful. A good one will be much more discerning & helpful.
This. My non-contact voltage tester broke after many many years of use, so a few weeks ago, I purchased this one from Amazon. It was inexpensive, but is extremely accurate. Also, there are multiple sensitivity levels.
I'm sure technique has something to do with it. Mine has a sensitivity adjustment and I poke it into an outlet, adjust sensitivity to what I like and then when I use it inside an open wall box, it's been reliable. Angle of attack makes a difference too. 
