How to determine if a string can be represented using a given encoding

I don't really like using exceptions for control flow, but the simplicity of this solution definitely beats creating a custom EncoderFallback:

public static bool CanBeEncoded(int codepage, string s)
{
    try
    {
        Encoding.GetEncoding(codepage,
                             EncoderFallback.ExceptionFallback,
                             DecoderFallback.ExceptionFallback).GetBytes(s);
        return true;
    }
    catch (EncoderFallbackException)
    {
        return false;
    }
}

Usage:

Console.WriteLine(CanBeEncoded(1252, "Grüß Gott!")); // Prints True
Console.WriteLine(CanBeEncoded(1252, "Привет"));     // Prints False

I solved this by encoding the string, decoding it, and then comparing it with the original. This seems terribly inefficient though.

Encoding targetEncoding = Encoding.GetEncoding(28595);
var text = "Гранит";

var encodedBytes = targetEncoding.GetBytes(text);
var decodedText = targetEncoding.GetString(encodedBytes);

var textCanBeRepresentedByTargetEncoding = decodedText.Equals(text);

Afaik, a String in c# is always Unicode. In this case you could cycle over every character of the string and check if its numeric value fits into a certain coding. E.g. a unicode character with 0x1234 will not fit into ASCII range 0x00-0xFF (0x7F, to be accurate).

EDIT
ASCII: 7 (8) bits. The "8th bit" characters are codepage-dependant, meaning the same numeric value will look appear as a different character in different codepages. No chance to change that, afaik.
UTF7: should be very rare, and according to wikipedia it's no part of the standard.
UTF8: 8 bits, identical to ASCII on the 1st half.
UTF16,32: 16 resp. 32 bits.
Afaik, the character 0x1234 is the same in UTF16 and 32, but of course does not exist in UTF8.
Unfortunately I don't know any way to find out if a given character 0xAB was given in ASCII (and in which codepage) or UTF8. Actually, I doubt that there is a way at all...