Better do your own research before you decided to join on ICO. You should read their whitepapper, team and roadmap criticaly. Because nobody can tell "this or that ICO is scam" before everything happened.
you're right, an investor based due diligence is KEY before jumping to buying any ICO.
If I were to assess an ICO, i'd def look into their business model, competitive analysis, problems discovered from their target market and how their products can solve those obstacles.