No explicit guidance is given as to how to tell what 'more potent' means

That means we go with what the words mean in 'normal' English. That's pretty clear here: you'd roll two bonus dice and use the higher number.

But wait! Actually, you explicitly do not do that! So, clearly something special is meant by 'most potent', but it's not defined anywhere.

At this juncture, many groups resort to various guesses at or specialized definitions about what 'most potent' might then mean. Some of the most common follow. Keep in mind that these are often used in conjunction, with one metric taking precedence and another coming in on ties, sometimes with many many layers:

• spell slot level

• caster level (i.e. the level of the character casting the spell)

• quantity of bonus or penalty added

  • variable bonuses and penalties are either treated as their average roll, or rolled and then considered, or their maximum roll. Sometimes this breaks the bless example

• quality of effect added (e.g. Aquatic Adaptation alter self is a bigger kind of deal than Change Appearance alter self, and so is 'more potent' so long as both retain their selected effects)

• remaining duration

• the one the player/PC picks

• the one that the DM guesses is worst for the player/PC

In the end, though, we don't know. All we can really know is that if you want to play with the errata trying to keep spells from meaningfully stacking with themselves, you've got to do a bit of work finishing the rules about that before you can play with them. Worth noting is that, should you decide that the spells are equally potent, then more-recent errata provides that the most recent spell takes precedence. That's only if you decide two spells are equipotent, though, and that depends on what potency means, as discussed above.