There’s no absolute way to know that PHP is crashing, but there may be signs. Typically, if you access a page that is always supposed to generate output (has a leading HTML block, for example), and suddenly get “Document contains no data” from your browser, it may mean that PHP crashes somewhere along the execution of the script. Another way to tell that PHP is crashing is by looking at the Apache error logs, and looking for SEGV (Apache 1.2) or Segmentation Fault (Apache 1.3). Sometimes due to segmentation faults, Phalcon could crash some of your web server processes.
Please help us to identify the problem by adding a crash backtrace to your bug report. Simply follow this guide to understand how to generate it.