Yes, any contract in which all the terms are legal and to which both parties entered in to freely is "fair".
Is there a chance of coercion in regards to whether one of the parties entered the contract? Of course there is and if that's the case, it's not a contract.
I don't know what an "unfair negotiating" advantage is, at least not as it regards whether or not I enter a contract, so I can't really weigh in on that.
The law in the UK literally disagrees with you about how this works. In fact our Unfair Contract Terms Act (see that name? Turns out even the people who named this act of parliament disagree with your idea that if it's legal that makes it fair, they figured if it's unfair let's make it illegal) says that if you "freely" agree to these unfair terms they still can't work.
You can see this principle at work in Britain's short lease (as distinct from a long leases of say 99 years) residential property laws too. A bunch of potential terms for such leases are defined to be unfair. Some landlords - after this change - went to court and said "well, but my tenants signed to say they knew the terms were unfair and agreed anyway - see, it's right here on page sixty of the contact" and judges told them where they could stick their unfair contracts. So sure enough those terms went away.
As to how you tell, courts already know this. If party A wrote the contract and party B's options were to agree or GTFO then party A has an advantage. Its normal contract law - oh, McDonalds signed this lease, but Tiny Co wrote it, so Tiny Co were the ones at advantage and anything unclear must not be construed to their benefit.
I don't think the UCTA disagrees with me. What it does is creates a framework that further defines what is legal / enforceable. Until it says that one party being unhappy with the outcome because they didn't read contract constitutes unfairness, then I think we're actually on the same page.
So it would be interesting if downvoters could, by way of reply, articulate how it is that contracts which are legal in all respects including all of the terms and the free will of those entering in to it, should be handled when they are deemed not to their liking after the fact.
If a large enough group of outsiders to the contract don't like the contract and can change the terms after the fact, wouldn't that undermine the confidence for people to enter in to them in the first place?
I haven't downvoted any comments here, but it's worth pointing out that the terms of service for any digital purchase are effectively impossible for most users to comprehensively read and understand. They're crazy long; I wish I could remember the estimate for how long each person would have to spend every year reading those agreements for all of their online services.
Anyway, people are free to be upset about terms and conditions even after they've "agreed" to them, particularly if those terms and conditions are widely recognized as onerous.
And, specifically to your point, courts have retroactively determined some conditions to be so flagrantly unfair that they've been stricken from contracts.
