Only on modern and/or high-end machines. On older more basic ones there's often a float of some sort on the salt filler cap, but sometimes not even that. If all else fails, it's quite a simple matter to dip a finger into the salt reservoir and taste it.
You don't need special salt for the dishwasher. Just use non-iodized salt. You can get it in a 1kg pack and it is more than enough for a year...
Only if you limit your use to about once a week. Pretty sure ours get refilled at least two or three times a year on average.
.... and the advantage of hunting around for non-iodized salt (and probably paying more for it than the cheap and widely available Reo Salt), would be ......?
most supermarkets do it. the glasses are much better with salt & rinse-aid
And if you don't want to pay for rinse aid, use vinegar.