Is there an alternative to saying "so-called" which doesn't suggest potential falsehood?

“Referred to as…” “Known as…”

I would use "what's known as", e.g. "Western countries belong to what's known as the Paris club"

The problem here, linguistically, is that any phrase which means this will take on the meaning of falsehood automatically, over time. It's the same way that any respectable word that means "has a disability" eventually comes to be an insult and then a slur.

If you want to say something like that, the word "putative" is still pretty unfettered by negative connotations, but only because few people use it. If it were in common use, it would follow the same path as "so-called". A more reliable approach in the long-term is to say what you mean using more words instead of fewer:

She could trust him more than any of her friends; although she wasn't sure those people were really her friends, it remained to be seen.

It's actually the length and awkwardness of the sentence structure that makes it resistant to misinterpretation.

I would just drop the "so-called" and put the term in quotes.

Western countries belonging to the "Paris club."

Putative: pyoo͞′tə-tĭv adjective Generally regarded as such; supposed. Commonly thought or deemed; supposed; reputed. Commonly believed or deemed to be the case; accepted by supposition rather than as a result of proof.

"as it's called" or "as they're called"? It's a bit wordier but I think conveys the meaning you're going for without the negative connotation. I also think context should usually direct a reader/listener to or from any negative connotation for the "so called" usage. That said, it doesn't hurt to be clearer.

"what is (generally/widely/typically/often) known as" comes to mind. Any variation of that would do the job.

It can also work not to use any term, but rather to introduce the name without fanfare. This implies a neutrality of judgment. "Western countries belonging to the 'Paris Club'." Even in speech one can often hear the introduction of a term by subtle changes in tone of voice.

alleged maybe? She could trust him more than any of her alleged friends.

aka

Hmm, every option I thought of has itself a dual-meaning.

Designated, denominated, entitled, dubbed. I suppose choose the one least likely to be confused for its homonym in the specific context.

I believe "per se" or "so to speak" can be used in that context too... per se

Western countries belonging to the Paris club so to speak.

Western Countries belonging to the “Paris Club.”

Using quotes indicates directly that this is being relayed as something someone else says or said.

It is factual and has less obvious shade than “so-called.” But someone could still say it can be used to cast shade:

My son’s “friend” stole his school lunch.

This is however irony, where you say the exact opposite of what is meant. So I’m not sure that counts.

But some hint of shade could be unavoidable since any time you report anything as something someone says, you offload accountability to a party not present, and invoke the possibility that that other party is fallible. Hearsay is inherently suspect. Why are you telling me what someone else said? Don’t you know yourself whether it is true?

But I don’t think using quotes necessarily jumps out as a way to mark something suspect.

Scientists have been searching for a “theory of everything”

the “Cambrian explosion” saw new forms of life evolve

he was enamored with “crew cab” trucks.

Pretty neutral. This might work, with the exception of deliberate irony.

She could trust him more than any of her apparent friends

Quote-and-quote?