How can i tell if a pineapple will be sweet?
Usually you can tell when Pineapples are ripen, which will be sweet, when


Leaves are loose and can easily be picked from the top
The shell has golden colour (but not dark brown, these are getting rotten)
The aroma even from out of the shell is strong


A typical pineapple that is bought green and unripen, takes one to two days to ripe well. Don't put them in the fridge if you want to ripe them. Keep them out on the counter. As a street food vendor, we always keep them in the truck for one night prior to selling, that ensure the pineapples ripe enough and the juice comes out of it.

Hope it helps.