Speaking in general terms, when required to conform FMC position accuracy eg before MNPS I use 2 or 3 DME stations.
Tune and ident DME station. Ask FMS how far to DME station and compare the result.
Repeat for another station (or 2) preferably one left of track and the other right of track to get a good spread.
Provided that the FMC and DME distances are not more than the allowed distance appart each time then everything is OK.
I only use VOR/DME if there is no other DME. The reason is that the VOR has a +/- bearing accuracy which at distance is far wortse than the DME accuracy.
Having said that to check the accuracy using VOR/DME I would go with "rudderrudderrat". provided that the indicated position (VOR/DME fix) is less than the maximum distance from the FMC indicated position, the FMC indicated position is accurate enough.
It should be possible to check distance from FMC posiiton to IRS position and GPS position etc so that when one is no longer in range of DME/DME position fixing one can expect that the GPS / IRS positions will do the job.