If it is commercial juice with nutrition info, calculate it from the label. Grams of sugar per serving (probably 240ml) will give % sugar, aka Brix. Multiply by 4 and 1000, then divide by 1000.
ex: 32 grams sugar per 240ml servine = 13.3 Brix *4 = 53, which = 1.053 OG.
Or, if there were 4 servings of the above in a quart bottle, you would add 128g of sugar, which is about 4.5oz, to your brew.
See the link below for typical values for a variety of fruits.