This is too simple!

People who did the hands-on work are listed by contribution from the front. People who supervised and funded the work are listed from the back. In the case where there is only one Principle Investigator (PI), they'll be last and the 2nd to last name will be the smallest hands-on contribution, but if there are multiple PIs (and this is not uncommon), the second-most-important advisory role will appear in the 2nd to last place.

So, lists of authors read like so:

IDid the MostWork,

Next was Me,

DidSome S. tuffToo,

...

IContributedExpertise and Funding,

ButIContributed the MostExpertise

And unless you read the affiliations (and sometimes not even then--you might have to look people up), you won't know where the breakpoint is.