What are the basic rules? Turn nothing with a crack? How can a newbie know? Haven’t seen anything in the books that tells me. Thanks.
Good rule of thumb when you are new - If you have to ask yourself "will this be safe to turn?" , you probably shouldn't try to turn it until you have some real practical experience under your belt ...
When you can turn something without getting catches to a form you want (practice), and have mastered getting nice smooth finish cuts that need less sanding, you should feel comfortable enough to go back to that piece of wood, and take a close look at where the defect is going to be, and ask yourself "if I get a catch here, would the blank break away, split, and fly apart?" by then you may have learned to read the grain and have had time to get familiar with your lathe, the tooling, the techniques, and more, and on riskier pieces, you may have solutions in mind to make turning it safer (Glue on a solid ring on the top, that can be turned away when done, or glue in scraps to bridge gaps or voids, etc, etc)
But as Dean mentions, you're best off finding a mentor to help you get over that learning curve faster (I still think of myself as a relative newbie, after 2 years or so of being self-taught... no mentors or clubs within reasonable drive times for me, unfortunately , so I have to do everything the hard way)