What you're doing actually works. Use small samples with refractometer to check if fermentation is done, knowing that (because of the presence of ethanol, as indicated above) it won't give you can actual reading, but in you can still verify that readings have become stable. Then use a hydrometer to actually read it.
There are devices out there using a different process to measure that CAN accurately measure with ethanol present using a similar amount as a refractometer (Anton Paar Easy Dens or their DMA35 are two examples, the latter more robust than the former), but they're pricy for most homebrewers (Easy Dens I think is a few hundred bucks, and depending on which DMA model that could be a few thousand).
So barring those, either a hydrometer, or a corrective calculator (Terrill and Novotny are the standard calculations)