Here is a nice - short but important - article on signs of dyslexia: Watch Out for These Red Flags These "red flags" help alert us to the need for further intervention and possible referral for full diagnostic testing.
For example, most PK-K students love engaging in nursery rhymes and rhyming games. A child who does not "get it" and enjoy this may be at risk.
At the elementary level, most people think that letter reversals or letters out of sequence in spelling may be a sign. Although this is true, think of it as a sound awareness issue - what is the sound difference between the /b/ and /d/ sounds, and what is the order of these sounds?
In middle school, is the student aware of common prefixes and suffixes, and can they chunk them to figure out unfamiliar words in science and social studies?
And don't forget that - even at the high school level -we still have to be on the alert to signs that may point to dyslexia. A review of a student's history may point to a reading disability even though it has not been diagnosed earlier.