$string = "In mathematics, summation (capital Greek sigma symbol: β) is the addition of a sequence of numbers."
$string.ToCharArray() | ForEach-Object{
$char = $_
try{$ascii = [byte][char]$char}
catch{"$char - None ascii character" }
}
Breaks the string into a char array and attempts is convert the char back to it ASCII value.
If it fails to convert then it is not a standard ASCII character.
If it fails to convert then it is not a standard ASCII character.
PS C:\> β - None ascii character