I recommend having a look at Glenys Hannell’s book Identifying Special Needs.
It has a variety of checklists that you can use from an observational perspective but also have the teacher fill in as well.
She published a new version last year…but the second edition is perfectly fine, and used copies are reasonably cheap on that ‘long river’ website we all spend a lot of time and money on!
Alternatively have a look for HumansNotRobots on the TES website. His checklists are free to download (although I think you need to ‘know’ what you are looking for a little more to use them effectively and they are a little out of date now.)