Omen (n) : a prophetic sign; something that happens that is regarded as a sign of how somebody or something will fare in the future. See also “portent”.
As fairly Plutonic person, I find that am perpetually seeing “signs” everywhere – it is somewhat of an occupational hazard, you might say, when this planet is active in your nativity. Things that might otherwise remain invisible to others tend to stand out to me as if they’ve been bathed in Luminol, LOL.
However, I realize that it can be challenging for many to work out exactly how to know if something is indeed an omen. That’s a tad hard for me to put into words, actually – when you’re naturally inclined to latch onto even the smallest of details and follow them like a proverbial breadcrumb trail and don’t believe that anything in life is a mere coincidence, synchronicity is found EVERYWHERE. Yet for the sake of this post, I will try anyway so that readers may understand – beyond a shadow of a doubt – exactly how to identify an omen.
So here is a related anecdote for you: Tonight on my way home from the gym, I saw one of these:

Cool, right?
But just two weeks (to the day) earlier, I had seen another:

Ok, a little weird, but still pretty cool.
What if I told you, though, that exactly three weeks before this instance – after emerging from a very profound healing session on the Gemini New Moon (that was conjunct Mars, BTW) – ANOTHER had appeared before me?

Yeah, that’s just SPOOKY! “Can you hear me NOW? Good!”
Always on a Tuesday, the day of the week ruled by Mars. And always lingering just long enough for me to get the shot before it literally vanished into thin air right in front of my eyes. Is it coincidence also that magical Neptune is currently affecting my (Plutonic-flavored) 8th House Mars, LOL? I don’t think you get much more magical than a rainbow, do you? Those are certainly neat correlations, but the primary takeaway for me lies in contemplating the symbolism of rainbows, which I always take as a harbinger of happiness after a difficult period (i.e. the preceding storm). Here is a quote from famed astrologer Isabel Hickey (rest her soul) regarding 8th house energies, taken directly from her fabulous book Astrology: A Cosmic Science – “Every 8th House operation is a celestial messenger in disguise and a challenge to penetrate this disguise and become the recipient of the blessing he bears. In the wake of an 8th House storm there is always a rainbow if we but lift our eyes to perceive it.”
So what’s the moral of this story? You cannot go wrong if you abide by the old Rule Of Three regarding omens! There is no way – in this writer’s humble opinion – if you encounter something via three separate instances, that it’s NOT a sign! This is roughly the equivalent of being hit in the head with a Cosmic 2 x 4, in my estimation. So if all else fails and you’re not sure if something is a sign or not? Watch how many more times it manifests – you’ll have yourself a definitive answer!

07/21/2015
OK so I have to vet this out, Dorothy. It’s a Lollipop Guild thing we Lollipoppers do [hikes up little trousers]. 1) How often do you take the routes that you snapped the pictures on? 2) Are the routes taken oriented with your back to the Sun at the normal time that you take them? and 3) Are rainstorms followed by clearing-skies late in the day normal for your local climate? Not that a “YES” to any/all of these makes the triple-rainbow sighting less awesome…not at all…it’s just that, wouldn’t some want to Virgo-map this type of thing out?!
LMAO – I’ll see your Virgo and raise you some Mercury…!
The routes taken were in the N/S directions, with the Sun on the western horizon somewhere between the 7th & 8th Houses. The rainbows, of course – as one might expect with the proper refractory angles required for manifesting this lovely phenomenon – were located above the eastern horizon. I travel these roads fairly often, but I am lucky if I see a rainbow once or twice a year ANYWHERE, let alone 3x in 5 weeks! The coolest part was seeing them linger just long enough to get my phone out and snap a pic since they dissipated within moments…almost as if they were hanging around for my benefit. I wouldn’t say storms with a late-day clearing are either particularly usual or unusual in these parts – this region does also get plenty of overnight or daytime soakers, but it just so happened a mid-afternoon storm followed by an evening clearing was exactly what happened in all 3 instances.
And yes, before you even ask, I’ve already worked out what was going on with asteroid Iris via natal, transit, and secondary progression. 😉