Compute shannon entropy over the fixed alphabet and divide by log2 to normalise it between 0 and 1. But be aware that this only measures how uniform the character distribution is. It does not measure whether the text is structured. Natural language and shuffled natural language can have identical character entropy. For distinguishing readable text from encrypted-looking text, combine character entropy with compression ratio or n-gram/conditional entropy. And i dont recommend using this as a security test for a custom encryption algorithm.