You can simplify this code to:
DirectorySearcher searcher = new DirectorySearcher();
searcher.Filter = "(&(objectCategory=user)(cn=steve.evans))";
SearchResultCollection results = searcher.FindAll();
if (results.Count == 1)
{
//do what you want to do
}
else if (results.Count == 0)
{
//user does not exist
}
else
{
//found more than one user
//something is wrong
}
If you can narrow down where the user is you can set searcher.SearchRoot to a specific OU that you know the user is under.
You should also use objectCategory instead of objectClass since objectCategory is indexed by default.
You should also consider searching on an attribute other than CN. For example it might make more sense to search on the username (sAMAccountName) since it's guaranteed to be unique.