If you're the hiring manager/interviewer, are they focused during the interview? Do they check their phone? Were they late to the interview (without a REALLY GOOD reason)? Are they not dressed appropriately for the interview (which could vary based on the industry)? Are there gaps in their resume that aren't explainable? Don't be afraid to ask some of these questions. If you think they have the skills, but still aren't sure, have a couple peers or other team members have a conversation with them too to gauge their reactions to the individual's knowledge, skills, abilities, etc.