How does one know if somebody is really declining an offer or just being polite?

人们怎样才能辨别某人确实是在谢绝邀请，还是只是出于礼貌才这么说？

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In fact, if done correctly, declining an offer today can help set up a future successful job search.

FORBES: Decline a Job Offer -- The Right Way!

However, he later resigned after declining an offer of redeployment within the authority.

BBC: Derrick Murphy