SmallBASIC syntax groups: 

Data function ISNUMBER

20  ISNUMBER (x)
Returns true if x is a number (or it can be converted to a number).

See examples

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? ISNUMBER(12)          :REM true
? ISNUMBER("12")        :REM true
? ISNUMBER("12E+2")     :REM true
? ISNUMBER("abc")       :REM false
? ISNUMBER("1+2")       :REM false
? ISNUMBER("int(2.4)")  :REM false