In the last game I played, one person remarked that the first to get air, is phantom.
Generally, it can be true for most cases; Mass VRs w/ carriers, fully upgraded, with Mommaship is very effective, as are mass mutas/corruptors, and mass BCs/Vikings, but I've seen phantoms build nothing but stalkers/marines and still win.
A good trick for finding the phantom later on, is see if they have units queued up, and not able to build. Then, when the phantom gets a supply boost, see if it changes, or if they start making more pylons/SDs, or overlords. Building mass of one building all of a sudden is another good indicator, or drastically falling behind in teching in an attempt to disguise.
For early on, I take into note who has unvisioned me or not. Some people will do it early on for fun, or to throw the real phantom off, but it's a good indicator sometimes. Also, if you're playing as a slayer or Paladin, and if you have a decent sized army, see which of your 'enemies' units counter yours best.
The last game I played, I was a paladin, and massed BCs, with thors for support. Then, I saw someone who's base was next to mine start to mass stalkers, DTs, and HTs. Stalkers for BCs, HTs for feedback, and I had no detection, so I got out 3 ravens. Turns out, they were the phantom.
I hope this helps people. :)