Well at the end of the day, there is no 100% way to tell if it will be shadowless. However you can be like 70% positive. @jkanly made an excellent article explaining the differences between the different boxes and which ones are the most likely to be shadowless and which ones are mostly likely to be unlimited.
Here is the link to the forum page.
www.elitefourum.com/t/identifying-shadowless-unlimited-base-set-boxes-packs/16993/1
Hope that helps answer your question a little bit better!
Mjisaacs