Hi Allison,

1) Most OS X software installs are drag-and-drop. If you're installing something simple and it comes as an installer rather than a drag and drop install you have to wonder why. I think twce before running any installer and would never run one I was offered by a gambling site or a porn site or similar.
2) No hole in OS X, just in the organic bit between the keyboard and the chair :)
3) There is no sure way to tell but generally the best thing is to stick to trusted publishers or to recommendations from trusted sources. It's a pure judgement call though, so no easy answer.

Ultimately we just need to addapt a more cautious attitude to installing software on the Mac. Keeping informed about the attacks that are out there is also a good idea so you know what in particular to be extra suspicious of.

Bart.