I was going to suggest using a level, but Denny beat me to it.
I'm no carpenter but I think you'd use a plumb bob. A shelf is level, a wall is plumb (and yes I know levels have a "plumb bubble" but since we're being technical)
But on the actual topic.
My father made a lot of fruit wines over the years. Sadly, he's not here to ask anymore but he made many batches of plum wine, especially wild plum wine (and wild grape and strawberry and orange and watermelon and ...). The fruit would be mashed and most of the pits removed (but not the skins) and metabisulfate added to kill the wild yeast. Then wine yeast would be pitched and it would ferment in an open fermenter for a week or so, covered with cheese cloth and then racked into glass for a long secondary.
I can't tell you more than that but when we were moving my mother from her house, there were probably 20 cases of plum wine in the basement and I tried some. Most of it would be at least 15 or 20 years old and it had the most amazing port profile of anything I've ever tasted. I dearly wish I'd been smarter and kept a few cases. It was fairly sweet (he'd add supplemental sugar) and with the slight oxidation and aging it was fantastic. It had a lot of sediment but with a careful pour you could get 98% of the wine out of the bottle. Stupid me tossed it all