Firstly, Mikao Usui was physically with his students when they received their attunements, and as he was the founder of this system of healing, I don’t ascribe to any sort of “over the internet” or distant attunements or trying to attune yourself. That would be the first way I would eliminate a course.
The second point for me would be to know something about the teacher. How long has he or she been using Reiki? What training was received in terms of learning to teach? (The traditional Japanese approach is a long apprenticeship over a number of years, until a point is reached where both teacher and student teacher believe the apprentice is ready to teach, and beyond the apprenticeship, they usually retain a mentor relationship). Unfortunately, some people only do a weekend, or a month, and then proclaim themselves ready to teach. As a formally trained classroom teacher with a 4 year Bachelor of Education Degree and many years of teaching behind me, I can assure you, that isn’t anywhere near long enough! *
It’s also good to know how many students might be present in an average class, and something about how the classes are conducted. For Reiki 1, a class with less than eight or so students simply doesn’t provide the student group with enough varied practise experience, interaction between students and teacher, and so on. A class that runs for less than 18 hours or so is probably lacking in depth and may not provide enough practise time for students to leave feeling assured that they are experiencing the energy we call Reiki.
There should also be a gap between completing Reiki 1 and II. Students need time to practise at the first level, developing awareness of the energy and discernment of subtle differences in terms of flow and sensation. While not being a hard and fast rule, I would recommend 3 months or so between completing the two levels.
I certainly do not support a single weekend where both levels are conducted as a single class. This format simply doesn’t give a student enough time for practise, let alone provide enough time to cover both levels appropriately and in depth. It was nearly a year between my Reiki 1 and II classes, and I believe this worked to my benefit in having gained quite a lot of hands-on experience before moving to the second level.
Finally, some basics – there should be 4 attunements at Reiki 1, and the teacher should have a verifiable lineage that goes back to Mikao Usui.
You should also check that the teacher belongs to a reputable teaching organisation and/or professional association.
*Please note, I’m not saying teachers of Reiki who have only done short courses are not caring and nurturing people – I am offering an opinion based on many years of experience regarding the minimum timeframe to develop understanding and expertise as a teacher (of any field of endeavour).