Some of the answer to your question of why the proper distinction between law and gospel is rare today may be found in factors like:
Charismatism, pietism, holiness competition, self-centeredness and self-focus, lordship salvationism, conditionalism, decisionism and plenty more like libertinism, anti-authoritarianism and anti-creedalism. These are all coming from various angles, legalist, antinomian and others, but they all do expose themselves when it comes to the critical issue of dividing law and gospel.
In more doubtful cases, look at the focus of the teaching. Is it always focusing on self, making you compare yourself to others, and is it always leaning towards "seeking" assurance in self (do I really have this and that -- things that you do have, if you are regenerate) and its experiences, then an alarm bell should ring.
A good and solid division on law and gospel will always be Christ centered and God-focused, and it will lead the flock closer to Him. Everything that leads you to ever look to self and to chase "experiences", how pious it may ever appear, you should be very careful with.