How can I tell whether a jackfruit has yellow or orange flesh by looking at the exterior of the jackfruit (i.e., without opening it)?

It will be a bit hit and miss because you don't have all the information required to know which variety it is - like the tree itself or even information from the shop the same way as you have with other plants (like apples, they're often labeled with the cultivar name e.g. royal gala, jonagold, red delicious, etc.)

You can check some cultivar guides, like this one, which is pretty comprehensive: https://www.growables.org/information/TropicalFruit/jackfruitvarieties.htm

This one has more pictures of the fruits, which might help when looking at the shelves: http://www.virtualherbarium.org/tropicalfruit/jackfruit-cultivars.html