Like a few have said before try to keep it light and positive.
Highlight things they do great give them some improvement areas then sandwich it with a positive. They are coming to us as a professional so we really have to do our job to reach them on their level and show them how we can help them without being negative or over bearing. I feel a lot of times trainers are so focused on doing what works in study but in actual practice every client is unique and needs a open minded approach.
What I like to do is if coming to you with weight loss goals and they are telling you say I’ve lost 5 pounds but I’m stuck at this weight. Highlight it HIGHLIGHT it grab a 5 pound weight have them stand while talking to them to show them that’s a significant amount while talking to them you can then see what else they are doing that might have caused them to plateau and here is your chance to give them guidance as a professional.
If they can relate to you and trust you as a trainer you can get much better message delivery without having them feel bad. The tricky part is building that rapport quickly and most of the times the assessment is the beginning so you have to be really unique in your approach.