Visual Basic Classic to check a text box for float

There are no way to differenciate an integer from a floating point (double) value since an integer can be stored in a double.
And forcing users to input the format you expect is not an option (users should not adapt to you).

At best, you can use the IsNumeric function (beware that it is subject to local settings), or use a regular expression:
^0*[1-9]\d*(?:[.,]\d+[1-9])?$     Check for numeric with a point or a coma as decimal separator
^0*[1-9]\d*(?:\.\d+[1-9])?$       Check for numeric with a point as decimal separator
^0*[1-9]\d*(?:,\d+[1-9])?$        Check for numeric with a coma as decimal separator

@Arana:
Still the same issue, not all floating point numbers have a decimal part.
Your functions fail when given a double without decimal part converted into string as parameter:

Dim Data As String
Data = cstr(cdbl(10))    '// double converted to string

debug.print IsFloat(Data)        '// return False

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Well unless Asker confirms otherwise I believe he wants to know if its a fractional number, because if he wants to check for a float then there is no way to verify it against a string made of numbers(its a string anyway), also float doesn't mean DOUBLE, unless you are refering to SQL Float, and there is no float data type in VB6, if you want the equivalent of C# Float then it will be VB SINGLE not DOUBLE.

CNT(10): 10 would fit in an int, why would you convert 10 to double? , and not a float for sure, unless you specifically assign it to a double data type.

if you want to check against a specific data type, and not only see if your value has a decimal portion then you can use
the vartype function you can use it to specifically check for 1 type like in:

Private Function IsSingle(ByVal value As Variant) As Boolean
    IsSingle =VarType(value) = vbSingle)
End Function

By the way, VB6 does not support regexp unless you call VBA library and all those regexp fail if your last digit is a zero. (ie 101.110) , I know 0 to the right shouldn't matter but as you said you cant force user to not send that as a parameter.