Yes to what blackcat said. Also, it helps to do the observation in conjunction with a parallel observation of a non-identified (as ADHD or any other need) same-sex peer in the classroom, so that you can reduce teacher and task effects.
Freely downloadable checklists that most pediatricians recognize: (Vanderbilt)
http://www.uwmedicine.org/neighborhood-clinics/Documents/03VanAssesScaleParent%20Infor.pdfhttp://www.nspeds.com/_files/Vanderbilt-Teacher-Initial.pdfA nice practical book on ADHD interventions:
"How to Reach and Teach ADD/ADHD Children." Sandra Rief.
Books on executive functions:
"Smart but Scattered". Peg Dawson
"Late, Lost, and Unprepared." Joyce Cooper-Kahn.