The stress cracks can be seen with your naked eye if they are severe enough, but, the ones that are just starting or are well hidden must be seen through a magnifying glass. They look like a horizontal line running around the tree trunk. How far around they run is how severe they are. Another way to see them is to put a very light pushing pressure on the green leaves of the tree and see if the cracks show up on the tree trunk. The crack will show up as a white horizontal line as the trunk separates. If you do not see a crack after that I would guess that your trees are fine.