I Use the Pearson Dyslexia Screening Test (DST-J) which is quick to administer and the reporting tool gives some ideas about support. However, it must be used alongside other assessments – especially observations of oral and written skills in the classroom.
As a specialist assessor, I use it as a way of targeting more detailed assessment of needs – e.g. phonological processing, working memory, reasoning, etc. – rather than as a stand alone tool – and I view the “diagnosis” that it offers carefully alongside other assessments.
More importantly, if you have a feeling that someone may have dyslexic traits, it is vital that they are taught appropriately. A multi-sensory approach is key so that all of their learning modes (auditory, visual, kinaesthetic, etc.) are engaged and that they have regular opportunities for over-learning and consolidation.