I found an excellent way to determine if a Qamets is a Gadol or Qatan, through the LHI interlinear. It is very clear in Genesis 1:21 that it is a Qatan. Now, beside this resource, if I came across a debatable vowel, is there another resource I can use? Do any of the Lexicons provide such information?
While I am at it, I wish to congratulate the author of the LHI for the accuracy of the work. It is really well done.
I should be more specific, and say that I am interested in ways to double check when the rules of Shewa are not clear for a specific case, as in theory it could be either. Most of the time, there is no problem just using the rules of Shwa to solve the questions.