headers_sent() will do what you want.

I know you're asking for a function which tells you if NON-header infomation has been sent (as stated in your second paragraph), but the exact way in which headers are buffered needs to be understood.

headers are never sent until content is forced out to the browser.  Therefore headers_sent() will not return TRUE until real content has reached the browser.

If output buffering is on, real content (such as: print "foo!\n"; ) can be in the buffer (but not sent to the browser) without having sent headers (and so headers_sent() will return FALSE).  When that buffer is flushed to the browser however, the headers are flushed as well (making that function return TRUE).

If output buffering is not on, individual headers can be sent, but headers_sent() will not yet return TRUE because header content is psuedo-buffered until content is sent.  Again, once content is output the headers are flushed and headers_sent() will begin returning TRUE.

Let's see if i'm getting the gist of this:
I send data to the browser, followed by a header, and headers_sent will return true.
Or something like that.

But what i want to be able to do is:

Check to see if any data has been sent to the browser BEFORE sending headers...
because the headers that I want to send are based on the content of the page.

i.e.:
Script to generate a .png image. Most browsers will only show a .png when the thing has the correct headers 
   Header("Content-type: image/png");

But let's say something goes wrong with the script, and an error is reported.
If i then proceed to tell the browser the thing is an image, i get a big white screen (because the result is actually a fairly messed up image, with text in it (which can be viewed by looking at the page-source))...

so my function would do something like:

if(proposed_function()){
   Header("Content-type: image/png"); //no errors
} else {
   //error, so tell the browser we're playing with text, not an image
   Header("Content-type: HTML");
}


I hope that better explains it... And if such an ability does excist (without having to use output control), then i'm sorry for wasting your time... :)

Wasting time.