First speak to people and if anything ear catching crops up a yellow triangle will appear allowing you to question them further. Second, you can move around environments by controlling your character's POV using the touch screen to find anything suspicious. Lastly, you can zoom in in some areas to view things in more detail.

Source:
• Scans from Neogaf Forums

Related Links:
• Official English Site
• Hotel Dusk, More Japanese Screenshots Revealed