For skin thickness: thick-skinned lemons have a specific appearance. They are larger, their pores are frequently larger, and their skin is somewhat lumpy. Pay attention to these criteria before and after you cut a lemon, and you will soon learn to recognize the difference between the very thick skinned lemons and the thin skinned ones.
As for the bitterness, I don't think there is a trick (or if there is, I don't know it). Make sure you are always using freshly squeezed juice, because either storing the squeezed juice or, to some extent, squeezing juice from a lemon that has been stored cut, makes the juice turn bitter. Also, the skin and the cell walls are bitter by themselves, so make sure you are squeezing properly, without getting too much pulp into the juice. But these are variations in bitterness related to the use of the lemon - as for the variations due to the lemon's grown amount of bitter compounds, I don't think you can know it beforehand.