How do you know when your mead is done fermenting? Is it when it clears? Is it when it is done bubbling? Is it when it reaches a certain specific gravity? How do you know that fermentation hasn't just stalled?
Specific gravity is your answer.
Once any indication of fermentation is complete i.e. it stops bubbling, then the recommendation is to take tests for gravity stability. You want 3 consecutive, identical gravity results (in the right area). Each individual test taken 2 or 3 days apart (at least), so the 3 tests have been taken across a period of about a week (minimum).
Yes, as long as all the kit used for the testing has been sanitised, I've never had any problems returning the sample to the main quantity of liquid.
Say for example, you wanted about 14% alcohol. Well that equates to about 103/104 points of gravity dropped. So if the must was made up to 1.110, then a stable gravity of 1.010 (1.007 for accuracy) would suggest that it can be considered finished.
Of course, generally speaking, the tolerance numbers quoted for most wine yeasts will have been worked out using grape musts and people often say that they've found that if a batch is properly nourished during the ferment, they have found that you can exceed the published tolerance by 1% or so.
Equally, to my mind, a must (using the example above) that was stable at 1.030 or higher, I'd consider as stuck and be looking at reasons why it had stalled like that.........
{edit} - oh, and it's also why it's considerably easier, when making "sweet" batches, that you don't use the beer making technique of putting all the fermentables in up front. It creates problems i.e. if you wanted to make an 18% batch - the gravity drop for that would be about 133 points, and if you wanted it to have, say, 20 points of residual sugar (presuming "finished" is at 1.000), then you'd be looking at a start gravity of 1.153 - which is up in the area of problem starts, potentially stressing the yeast, etc etc.
It's far easier to start lower, but using nutrients for the full alcohol target, then step feeding the batch honey to the appropriate level.