I have never seen Bombus suckleyi in the wild. I have grave concerns that I never will. Suckley cuckoo bumble bees seemed to prefer the Western bumble bee, Bombus occidentalis. When occidentalis populations crashed, the preferred target species was no longer available to suckleyi.
Predator/prey ratios suggest a smaller population of suckleyi relative to their occidentalist hosts. Suckleyi numbers probably also plummeted, but their population base level had never been estimated so we have no baseline. What we do have is years of increased bumble bee sampling and very few detections of suckleyi since the late 1990s.
If you think you see one, please get good pictures but give serious consideration to NOT COLLECTING IT. Any individual you encounter is a sexually active one. As cuckoos, there are no workers, only queens and drones. At the current population level we need to help all survivors bee successful.
Looking at the color forms for Suckleyi shown in Bumble Bees of the Western United States, all forms of the Suckley bumble bee have a solid yellow T4. That easily separates it from the Indiscriminate cuckoo bumble bee.
Also, male B. suckleyi are all or mainly yellow. Other cuckoo males are not. So if you see a yellow cuckoo you have something special! Get that picture!
The key distinguishing photographically useful feature for separating B. suckleyi from B. insularis and B. fervidus, according to the key in Bumble Bees of the Western United States, by Koch, et al, 2012, is the color of the hairs on the back of the head (Occiput). I have used this in a key, with a handlens, but may be a challenge with just a camera photo.
If the hairs on the occiput are predominantly black it is B. suckleyi.
Both B. flavidus and B. insularis have mainly yellows hairs on the back of their heads. This is important to remember, as far as capturing the key elements needed for photographic identification. Make that shot!
But please, do not collect it!