If they are low enough you can grab and feel them, and you are not worried about animals ... then the best thing is wait till they are a little soft. The scratch the skin trick works too, I just hate scratching my fruits.
If you have a super huge tree like me, then you need other tricks. I try to eyeball the size, but that is not reliable. Sometimes small ones are ripe and something big ones are not ripe. Sometimes I shake the branches too see if they will just fall on their own. Sometimes I give the fruit a light tug with a picker, if it comes off with a light tug, then it probably was ripe enough.
And sometimes I just get it plain wrong, and oh well I picked an unripe Sapodilla