The amperage is only flowing when the starter is turning.  Otherwise the only amperage that will flow is feeding various electronic devices that might happen to be on.  Turning the key off will help keep this to a minimum.  The only spark you might see will be a tiny one unlikely to cause damage.  If the polarity is reversed there will be a shower of sparks similar to welding as the current will be about the same.  Serious damage or injury can occur.  Quickly striking the clamp against the terminal to check polarity will work but on modern vehicles may be enough to damage something expensive.