Related question
What should a good diagnostic methodology actually look like?
Short answer
It should have design rules you can inspect. Every question should serve more than one purpose, the set should mix qualitative and quantitative signals, nothing should feel threatening enough to make people guard their answers, and there should be a defined method for turning what is collected into a comparable result.
Most assessments are questionnaires with a logo on them. The difference between a questionnaire and a diagnostic is whether anyone designed the constraints before writing the questions.
The most important rule is that the questions you most need answered usually cannot be asked directly. Ask a business executive whether his company is considering relocating and the meeting ends. Ask a department head whether his team is quietly working around the official system and you get a careful answer. The information has to come out of conversation rather than confession, which means each question has to earn its place by serving more than one underlying objective.
The second rule is a time limit. We held ours to 45 minutes, because past that point we found we were collecting polite answers rather than better ones. People manage what they say when they are tired.
The third rule is that collecting is not analyzing. This is where most assessments stop. If a methodology gathers a hundred pieces of information and hands you a narrative summary, someone's judgment is doing all the work and you cannot compare this year's answer to last year's, or one department to another. A real methodology defines in advance how collected inputs produce a result you can hold up against something else.
Those rules are not theoretical for us. We built a diagnostic on exactly that design, ran more than 110 collected inputs per interview through a scoring model, eventually patented the analytical process, and operated it for 22 years across more than 1,700 organizations. The subject was different. The design problem was identical.
When you evaluate any assessment, ask the firm to explain its design rules. If there are none, you are buying a conversation.