Are these concentrates or just pre-mixed juices? Most pre-mixed juices are not the greatest for wine making. They are not really "Full Strength" but often watered down to some degree or mixed with other juices. (Typically a juice like peach or cherry might also have apple or white grape juice mixed in with it as those are much less expensive juices. ) With concentrates you still need to check out the list of ingredients - you might be surprised,.
Certainly you can make wine 'on the cheap' using such juices but you may find the flavor a little too light on the palate.
One great example of the pre-mixed juices is the Ocean Spray juice selection. Many are Cranberry and some other juice Like Cranberry cherry (I've posted a copy of the nutritional information below) - BUT on closer inspection you will find that the list of ingredients is different than what the label would lead you to believe. Cranberry is not even the leading ingredient.
So - First thing is to read that list of ingredients VERY VERY Carefully. That should be no problem if you are buying them in person other wise if buying online - check it out, email or call seller/manufacturer if need be.
If you are using a juice like say bottled Tart Cherry Juice concentrate - most often the will provide you mixing instructions per serving and the calories per-serving. That's critical information to have. Generally I like to go the route of making my wines VERY full bodied. Example: 16oz Tart Cherry Juice concentrate per instructions says it will make 16 x 8oz servings (128oz/1gallon) When using that concentrate I use 4 bottles to make 3 gallons of wine. and actually for Tart Cherry I use 3 tart cherry and 1 sweet cherry)
So You really can't just go by the hydrometer - difference fruits have different amounts of sugar in them. (See this thread for more info:
Sugar content of Fruit ) You will have to know what is in them and taste it to see how strong the fruit flavor is in that particular juice.
View attachment 68561