string ComputeHash(string truc)
{
    using (var sha = SHA256.Create())
    {
        byte[] bytes = sha.ComputeHash(Encoding.UTF8.GetBytes(truc));
        return System.Convert.ToBase64String(bytes);
    }
}

string json = JsonUtility.ToJson(data);
string newHash = ComputeHash(json);

if (newHash != lastSavedHash)
{
    File.WriteAllText(path, json);
    lastSavedHash = newHash;
}