Why would you be logging it as dual?
If you are otherwise current, a check flight is PIC time not dual, right?
No. The operator of the aircraft nominates the pilot in command for the flight. If the operator requires you to complete a check flight, then
of course you are not nominated as being in command - as the very purpose of the check is to see if you are suitable to command in your own right!
Being licensed and qualified doesn't mean that any time you are in the aircraft you log command time! A 737 Captain filling-in in the right hand seat on a line flight logs the time in the co-pilot column, because that is the role they are filling. In this specific case (as a 90 day check) you would not be qualified to command a flight with a passenger in any case.
The instructor or check pilot is therefore in command, and only the commander may log command time. This remaining options are dual instruction, or ICUS (which, if you don't have a specific ICUS column would be logged in the co-pilot column with an explanatory note that the flight was ICUS).
If the flight is to be logged as ICUS, it must be done with the approval of the commander AND the operator, and briefed as such.
(As an aside, I once completed a check flight at RVAC with an ex-girlfriend of mine who was working there as an instructor. The flight was in a two seat Eagle. My logbook shows her name as the PIC, and the flight is logged in the dual,single column: my having ten times her experience had nothing to do with it. It was a nice turn-around as I had been one of her instructors when
I was at Royal Vic
)