Matt Humphrey is completely correct. However as an additional check to
the heuristic of looking for unprintable characters, another trick is to
check if the newline string is consistent. It should always be either "\n"
(for UNIX-like systems), "\r" (for Mac-like systems) or "\r\n" (for
Windows-like systems). If the file starts switching around between these, it
probably isn't a valid ASCII file on any of the above three platforms.

   You could also disregard 2-byte UNICODE characters as being "non-ASCII",
and lump them in with the category of "binary files".

   - Oliver

To see if a url is binary or not without relying on the header.

I'm using something like this:

protected boolean isBinary(String url){

  boolean isbin=false;
  java.io.InputStream in=null;

try{
   URL bin_url = new URL(url);

   in = bin_url.openStream();
    BufferedReader r = new BufferedReader(new InputStreamReader(in));

    char [] cc= new char[255]; //do a peek
    r.read(cc,0,255);

    double prob_bin=0;

    for(int i=0; i<cc.length; i++){
     int j = (int)cc[i];

     if(j<32 || j>127){ //with chinese and other type languages it might
flag them as binary - need another check ideaaly
      prob_bin++;
     }

    }

    double pb = prob_bin/255;
    if(pb>0.5){
    // System.out.println("probably binary at "+pb);
     isbin= true;
    }

    }

    in.close();

 }catch(Exception ee){
  System.out.println("WARN! Couldn't find isBinary() content-"+url);
  isbin= false; //error - likely broken link - so return false
 }

 try{
  in.close();
 }catch(Exception E){}

 System.out.println("url isBinary():"+url+":"+isbin);
 return isbin;

 }

I read somewhere that finding \n's might work as well.

Also, are ASCII 7bit and binary 8bit or something? Is there a way to find
this out - like analyse a byte?