If a person asked me for specific deck advice in regards to their playing style, I'd ask them first if they had gotten their deck off of someone else (say, if they were using a derivative of someone's mono-fire deck or a popular (SG, PuppyChow) rainbow), and if they did, proceed to look at their cards as well as how many they were using (in pve and pvp), and then also how many were upgraded. I'd also take a look at their mark.
Then, I'd ask what style they were attempting to use - pillar/quanta denial, creature/permanent control, poisons, anti-deck out strategies, monster rushing, spell use, etc. After all of that, then I'd either suggest to get rid or add a few cards in regards to what they want or need, and possibly make a deck similar to theirs and show them a link/screen shot of what it would look like, while also checking to see how closely it relates to the original deck they copied from. In addition, I would also suggest that they would get a different mark if it wasn't efficient.
If they're inventing a new style, I'd personally would use their deck for a little bit (assuming that there are no glaring mistakes, such as a mono-aether deck with no aether pillars and aether mark), and then figure out how to reduce the weaknesses (adding a few PA/EA to protect permanents that they rely on greatly, removing excessive monsters and replacing it with a fractal or other creatures to compensate if they're trying to rush, etc.). I'd give them my opinion about it after playing a few random battles against the computer, and possibly a false god or another player to check.
Although your question is somewhat vague baku36, the above two paragraphs explain my general approach should someone ask me that question. ^^
