Perhaps it’s not so much distinguishing what is a sign and what isn’t, but rather how in tune your mental kaleidoscope is to being able to see the underlying pattern & meaning to everything. In other words, theoretically, if the universe is holographic through-and-through, then any part of it should be able to tell us something about any other part. It’s just a matter of how skilled we are at peering into the fabric of space-time and picking out the underlying weave. 
So HOW do we start seeing omens and portents in our daily life? I can’t remember where I first came across this, but one useful exercise for increasing the level of synchronicity and frequency of omens in your life is the following:
Set aside 5 minutes a day for this, just like you’re going to perform a daily meditation. Sit somewhere comfortable (outdoors, indoors, doesn’t matter), and just look at everything around you. Give every object in your environment incredible meaning and purpose. Know that there’s a reason why that hummingbird circled three times around the feeder before drinking. Know that the placement of books on your nightside table is trying to tell you something important. Know that there’s something deeply significant about the number of 7’s in the phone number of the last telemarketing call that you received, or the pattern of car honks you just heard during your morning commute. There is nothing random or meaningless about these things. They are all part of an underlying pattern and you just have to figure out what it is.
If you perform this exercise on a routine basis, pretty soon you will notice that meaningful “coincidences” will start popping up more and more frequently, which will turn into obvious synchronicities, which will turn into useful omens if you keep at it long enough.
Another way to accomplish this is through a method found in Tibetan Dream Yoga. The idea is that your dream state is replete with symbols and metaphors for various messages that your subconscious is trying to tell you. By keeping a dream journal and noticing which symbols frequently pop up, you can map out the architecture of your psyche and eventually wake up within the dream itself, which is called lucid dreaming.
Now, just apply this same technique to waking life. Treat your waking state exactly as if it were a dream: look at what symbols pop up frequently. Maybe it’s that you’ve been constantly seeing ravens. Or perhaps the number five keeps showing up everywhere you turn. Keep a record of these events in a journal, or at the very least make a mental note of their occurrence and possible meaning, and over time you’ll start noticing patterns. These frequently noticed symbols are archetypal omens, whose knowledge and use of can help bring you one step closer to “waking” from normal unenlightened reality.
Warning! Do not attempt these methods if you have a history of psychotic breaks or mental illness.
As I said in my OP, these magical states of synchronicity can be perilously close to schizophrenia. Don’t take my word for it, though… From The Eden Express, a memoir by the schizophrenic son of Kurt Vonnegut: