"Is it offensive?" is the WrongTM question.
"Is this offensive" is the Wrong question to ask.
The right question is "Will this offend people?". Good supplementary questions include "Will this offend anyone I care about offending?", "Will this offend anyone in ways I care about?" and "Will this upset anyone?"
"Offensive" is *not* an objective property that exists in a vacuum. Any statement that offends people is, ipse facto, offensive, and and statement that does not is not.
When deciding whether or not to say something, I think that asking "is this offensive?" is likely to result in you causing offense more often than asking "is this likely to cause offense?", because human nature is to respond to the first question be trying to find a line of reasoning as to why it isn't, whereas the second will result in actually thinking about how real people will respond.
If someone takes offense at something then telling them that it is not, in fact, offensive, is just silly. Sensible responses include "I do not care about offending you", "I do not care about offending you, and also think you are silly to have taken offense" (c.f. the recent spate of offence at an interracial couple in a cereal advert), "I am sorry to have offended you, and withdraw the remark" and "I regret having offended you, but nevertheless stand by the remark because while I respect your judgement in general I think you are wrong in this case".
But "It wasn't offensive" instantly and automatically puts you in the wrong. Any remark that someone has taken offense at was offensive by definition.