When at a RES site and wanting to hunt large ship targets (clippers, pythons, anacondas, dropships, etc) I had found good advice from this
forum thread. But to summarize the procedure for checking ship spawns:
step 1) go to RES site
step 2) if the first wanted ship you see is
NOT of the size of ship you want to fight, skip to step 4, else continue to step 3
step 3) if the second and third wanted ship you see is the correct size of ship, congratulations, you have gotten the correct spawn type and as long as you don't leave RES site or have another player join your instance the spawn type should not change, thus you can keep hunting and exit this procedure. If the second or third ship are not the correct type, proceed to step 4
step 4) open menu, select save and exit to main menu (do not select save and exit), this is faster than entering and exiting supercruise and causes less wear and tear on ship
step 5) start the type of session you were in again (solo, open, or private group, but solo is preferable for this procedure)
step 6) repeat step 2 (continue repeating steps 2-6 until you are tired of this or successfully exit on step 3)