There are different ways to find that out.
1. If you know the Q factor of the coil (Qcoil), you can directly find out Qck through the above measurement. It is better to find that out through several coils with different Q, which may also give you an err margin. Save this capacitor for future reference.
2. Use capacitors with very high Q factors, such as air filled capacitors, or single layer capacitor with Qck>3000 at say 1 GHz. When the Qck is very high, you can verify that when you use Qck=3000, or Qck=6000, there is only small influence on your final results. If you use air filled capacitor, you can easily assume Qck=5000, or above, which has little influence to your final results.
3. When you do more experiments, you will know the approximate range of Qs of some capacitors, and use these capacitors as your reference.
My advice is try several type of capacitors, and parts from different vendors, as your reference capacitor.
Good luck!