In order to get high quality video for your project, before you hire, (once you've established they're licensed and insured), contractors should look for a number of skills that drone operators must have. Use their own previous content from their website or YouTube channel in order to judge their abilities and look out for these issues....
1) Smooth movement.
When you watch drone footage back, keep your eyes on one moving object. Sheep in fields are great as they stand out. Does the object glide smoothly across the scene, or is it jerky? Smooth panning only happens when a number of things are set up correctly. Whilst some of the problems can be caused by playback devices (iPads are great for having smooth playback) a lot of the time, it can be because the film has been rendered and filmed in the wrong or different formats. A great drone operator will already know how their drone films best, and what formats are the best for producing. This can also be down to poor piloting skills though. Sometimes a shot is left in a final edit (that should have been cut) when the drone pilot simply changed direction of their drone or camera suddenly.
2) Consistent colour and brightness
Beginner drone operators will likely pop their video settings on auto. This can cause a number of problems when creating video content. As the light from a moving drone is always changing, so too will the camera settings. This can be identified by regular changes in the colour or brightness, as the camera will use it's ISO setting to adjust the brightness for the scene. One encoded onto the video, this is impossible to smooth out. You can spot this with regular step changes in the scene as it retains the correct exposure.
3) Overblown whites or too much black
Photography, and especially exposure is something of an art. Getting the ISO and shutter balance right for a scene can be difficult. Once footage is acquired though, with the right professional software, a lot of detail can be pulled out from a correctly exposed image. However, it's not always easy and sometimes (when a shot is moving) a complex adjustment will be necessary to ensure the exposure is right, and also smooth if it changes throughout a scene.
The final thought for your drone operator is how well they frame all of their shots. Several rules about diagonals and thirds apply to video as much as they do photography. And while some drone pilots may be great photographers, framing a shot with a drone is much more difficult. Having the ability to adjust the altitude, attitude, movement and camera angle whilst maintaining a satisfying image is a skill that some people just can't get right (in the same way that I'm unlikely to ever be able to skip, or hula hoop).
5) No fly, no fee
Finally, before you book someone, check what their policy is for bad weather. It is common to book a pilot for a day and if the weather prohibits a flight, then they still charge the client. This is common practice in this sector.
If you've seen a pilots example videos of their work and they don't have any of these flaws, then you're safe to assume they know their kit and will be able to get you high quality video that you deserve. If you see these issues, on recent videos they've made (we made these mistake we we were new) then perhaps look elsewhere.