A character is considered 'adept' at a spell or skill when it reaches the maximum percentage at which it can utilize the skill. The default maximum is 95%. However, not all classes will be able to achieve this level of efficiency in all skills. Current and maximum percentages can be seen via the 'slist' command. Also, you will receive a message and an experience bonus when your character becomes adept at a skill. The skill percentage will then be colored gray when viewed via the 'practice' command.
Note: Even having a 100% efficiency in a skill does not mean it will work every single time. Other factors (magic resistance, negative affects, etc.) can reduce the effective efficiency of a skill and cause it to fail.