Some of those standards of observation are in the PDF below:

You might want to download it and take it into consideration, because one of my sources says a key feature of omens is, true omens are empirical in nature. Their significance has been drawn as the result of centuries of observation and subsequent events. One notable example of this is comets. Time and again, comets have been observed to precede a war or some significant conflict. One comet’s appearance was considered so significant, it was included in the Bayeux Tapestry about the Norman invasion of England in 1066 C.E., where a comet is seen flying over the hapless King Harold’s head. It’s happened often-enough in history that the next time you see a comet, count on it, two or more countries are going at it in the near-future.

Omens are often either out-of-the-ordinary behavior or something which simply doesn’t normally happen. A squirrel storing nuts in the autumn isn’t odd; seeing it similarly-busy in the spring might be a cause for concern.

Omens are things which are not explainable any other way. A body-itch omen could easily be a case of dry skin. More challenging is something like finding a gold bracelet on your patio. It’s probably not an omen if it was dropped by a family member, thrown by a neighbor in the midst of an argument, or unwittingly dropped by a visitor. It’s probably not an omen if you were wishing for a gold bracelet, because that’s the ‘pizza’ part of the ‘omen or pizza’ dichotomy. You ordered it, so to speak. “Once you have eliminated the impossible, the improbable, however unlikely, must be the truth” stated Sir Arthur Conan Doyle’s Sherlock Holmes. If you have eliminated all other likely explanations, then what you have on your hands may well be an omen. It may’ve been dropped by a Raven or a Crow. The Corvus Family has a reputation for being messengers from the Gods and in and of themselves, have been considered omens.

Omens are those events which make you look at each other and say, “Now THAT was weird.” You’re parked at a stop light, waiting for the light to change, minding your own business, and suddenly two birds decide to get into a fight right there on the front end of your car. Might be a Sign.

Omens often simply feel significant. It can manifest in a couple of ways. Hackles rising. Goose flesh. An inner knowing centered in your head or solar plexus. A simple gut-hunch. But sometimes, there is an instant physical reaction to an omen.  You just know it’s a Sign.

The two principles immediately-above are your most-likely reaction if it’s an omen which doesn’t appear in any official list of established omens, such as the PDF posted above. Sometimes, omens are specifically tailored by God/Universe/Other Parties for you and you alone.

Omens are something you can ask for. It’s not just for the College of Augurs. You can pose a question or issue, then say ‘send me an omen.’ Your dead relatives, spirit guides or guardian angel could be champing at the bit to send you a message, and all they need is an invitation. The challenge, of course, is remembering you asked and seeing the resultant omen for what it is. It could come in three minutes, several hours, or take a whole day to manifest.