Yup, harder to identify trends accross subjects. Unless there is an obvious need made clear immediately it can take a couple of years to put together the pieces accross staff, especially for dyslexics that have developed good coping strategies.
So, at our school to try and pick up issues we do two things with year seven initially.
CATS (where we can see spikey profiles, sometimes less obvious with very able dyslexics though) and a reading test (which tests single word, sentence reading and comprehension). These two sets of tests often filter out those who merit further testing, which is done by our qualified exams access arrangement assessor. She does various tests more reading, more comprehension, sequencing, processing, handwriting. Then, the results are fed to the SENCO to share with staff as appropriate. If the EAA and SENCO (or parents) think that there is likely to be an underlying issue we contact parents to ask them to refer to local dyslexia charity who are qualified to assess and diagnose (much quicker than ed psyche as they are like hen's teeth!). That process costs £350 though. School pays for PP children's assessment, most other parents pay but if they say they are going to struggle we put them on the long waiting list for an LA Ed psyche to do the assessment.
This system picks up most students with dyslexia/dyspraxia and helps us identify and support the very weakest appropriately but doesn't often "catch" academically able dyslexics etc.
We have a back up though!
In year nine ALL students are assessed for exams access arrangements. Everyone starts with a reading test they then do more tests if they score low (not just low for their age but also if we feel it's low for what we know of them as a student). Often this is where the academically able are picked up as the gaps tend to have widened a bit at it's easier to identify patterns that don't match (so, between test scores and CAT scores etc.). Then we follow the same system of referral with them as we do with Y7s.
So, at our school we think we do pretty well with identification HOWEVER we are lucky to have an extremely qualified assessor and a relatively small school roll, meaning that the time taken for all this testing does not impact as much as it could on a school with an intake of 200+. We're also very lucky that on the whole we have parents who engage willingly with us when we contact them - and that so many of those parents find a way to pay for the assessment. Perhaps that's because our system is so robust that if we identify something concerning they trust us that it's money well spent or perhaps it's because they have had concerns themselves? Either way, I count my blessings!