Tact and maybe asking directly with a PM? I have sent someone a PM first asking if they want some pointers or tips, especially if it is a model I have built and have some knowledge of. Am I a pro builder no. This is a tough line with a forum or even a contest. I run into this with the real 1:1 cars, people ask to have a car judged, or what is wrong with their car, and then are all bent out of shape when they are told something is not right.
With a lesser quality build, tact is key. People need to offer suggestions on what would make it better without criticizing the person themselves. I still run into new builders at shows that will ask about BMF, or where did some of the detail items come from. It amazes me if you talk to people at show, not the ones you know already, but the people looking at particular details on kits, that they may have never been to an NNL or large show where they find out what is out there. You can get great conversations going and sometimes get someone to another event and that gets them excited and trying to build better. Telling anyone their work is bad or junk never helps anyone.