From a coach's standpoint, it's very easy. If my team is on defense, then the call should be a charge. If my team is on offense, then all calls should be blocks.
It takes officials to make a simple rule like that complicated.
Seriously, what Travelin Man has said is good advice and is consistent with many of the other officials here. Watch the defense and you'll see the positioning and timing of the players better both prior to and as the contact is occuring. That increases the likelihood of making the correct call.
In fact, this little tip of following the D has helped me become a better coach, seeing the play develop better and giving me clearer information on what we needed to work on as a team.