You don't have to be a master screenwriter or top director or producer to know you are reading a good script. A good script should be a page turner. You should feel compelled to want to keep reading more when you reach the end of a page. As you are reading the script, actual scenes should start playing out in your head. You are seeing the film, not just reading it. If this happens, it is a strong clue you are reading something that is well written.
However, every well written script won't speak to everyone. I have read many that are award-winning that I felt weren't my cup of tea. That doesn't mean they are bad. It just means that they aren't my taste. Sometimes it can be hard to discern between a bad script and one that just doesn't speak to you. This is where having a firm grasp on script structure can really help as you assess the strengths of a script -- read up on script structure, take classes, watch a ton of movies and read the scripts of the movies you like.
I won't lie, reading scripts can be a chore at times. There are many more bad scripts than there are good ones. Because there are so many scripts available and not enough time to read them all, it is a reality that development executives use time-saving means for getting through mountains of scripts each week. I hate to say it but there is a 30-10 rule. Read the first 30 pages and the last 10. And there is also the "only read the dialogue" way of reading a script. If the script is really bad it's hard to get past page 3 and yes, I have passed on scripts from reading only 3 pages. I know that may sound insane but because I have so many other scripts to consider, I would rather not waste my time on a script that has tortured me for 3 pages. Don't worry, a script has to be really bad for me to pass after 3 pages. Really bad.
I am sure screenwriters are cringing as they read this. They want to believe that their scripts are being read from beginning to end and that the readers are savoring every word. And that can be a reality if you have crafted a really good script. So hopefully knowing there is a lot of stiff competition out there, screenwriters will feel compelled to make sure their script is so good that an exec will keep reading the entire script. Keep those pages turning so fast that before an exec even thinks about using the 30-10 rule, he or she has already finished reading the entire script!