• In a terminal running Sage in command line, type x.simplify<TAB> where <TAB> is the tabulation key (usually left of the Q key in a QWERTY keyboard). This asks Sage to "auto-complete" your command (in your case, all the methods of x (a symbolic expression) whose name begins with simplify) : it will give you the 9 (in Sage 9.2.beta5) possible choices. canonicalize_radical is an oddball, replacing a former simplify_radical that was (mathematically) too ... radical (long story).
• The documentation, ... well ... documents various ways of searching and perusing the online documentation (both on your machine and on the Web). You may Google Sagemath documentation, or start here.