Interesting Dispel Magic Questions

[Blatant Beast]

The answer is simple: the nature of the spell effect is described in the spell text. Alter self, for instance, clearly says "you adapt your body" and "you transform your appearance" and "you grow claws, fangs, spines, horns, or a different natural weapon". Clearly the spell effect resides on the caster (i.e. a creature.)

That is not a simple answer to enact in real play. The answer boils down to "use your own judgment" which means there is effectively no standard.
Your Test is to eyeball it, because you think the answer is clear. Clearly, the potential answers are not clear, if the answers were clear this thread would not exist. Multiple people have already commented that your judgement on how to adjudicate the Spirit Guardians spell vis a vis Dispel Magic, does not match their own.

As someone else already stated, there is not going to be a definitive RAW answer, because the rules are vague. So the discussion hinges upon what is the best outcome for the system, as the point of principle judgement.

5e's Dispel Magic, itself is not clearly written. Magical Effects as a category, encompass more than just spells. Dispel Magic could be read, that spells under 3rd level are canceled, spells greater than 3rd level receive a saving throw. Since, no non-spell based magical effects are not mentioned as having a saving throw, then those effects are automatically dispelled. 5e even has precedent for this type of interpretation, unattended objects generally do not receive a chance to make a Saving Throw, (which is why the Artillerist's Eldritch Cannon has a separate and specifically created category of magical object).

It is only Sage Advice that explicitly mentions that Dispel Magic is limited to only affecting spells. Dispel Magic's spell write leaves it as inference that only spells can be targeted, through not mentioning a saving throw mechanism, but that is certainly not a definitive conclusion from the text as written. A quick perusal of many threads on this board, and elsewhere on the internet demonstrates that 5e's tendency to remain silent on the specifics leave the decision up to the DM, creates a multitude of interpretations that are vigorously defended, and the answer seems to be "ask your DM".

Honestly, I use 2e AD&D's version of Dispel Magic as my house-ruled substitution for the 5e PHB version, and I do not regret it. 5e's is just a shoddy mess, in quite a few areas.