Very good question. It's very easy to be deceived and they will try to do so. But all you have to do is ask. I once thought I was talking to God, but noticed that something was slightly off. Luckily I thought to ask, "wait, how do I know you're God?" That instant it became very clear that it was not God. Scariest moment of my life.

I didn't know this "rule" when it happened because I didn't know Christ at the time. But the Bible gives specific instructions on this, so I'd go with those:

"By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God, 3 and every spirit that does not confess Jesus is not from God. This is the spirit of the antichrist, which you heard was coming and now is in the world already."

By the way, protip, after physical death, when you feel drawn in by what you perceive as brightness, make sure you ask that brightness if he's really God. Lucifer appears bright, so don't be bashful about questioning God's identity as he's letting you in to Heaven.