As far as I know there is no written test, I believe there is a ride in the actual aircraft if they have one available. Just like a .8 flight to do a couple of approaches to make sure you aren't a complete dumb-dumb -- they're not looking for straight on ATP level PTS on an airplane you haven't flown before, just an assurance that you are in fact an actual pilot (Baron got burned about a decade an a half ago by a guy who was faking, but apparently it was one of those flightsim pilots who couldn't fly an actual simulator and he bombed out of flight safety) and that you can pass a checkride in the sim after a 10 hours of training -- looking to see if you're trainable -- pilot training costs, while paid by FedEx, are one of the metrics that is compared between the feeder family so there is a healthy and vigorous competition to keep that low.
Caravan flies exactly like a 182 that sits a little higher though... I don't know about WestAir but I think the last time someone interviewed and bombed the flight eval was like 4 years ago here at Baron. Doesn't happen very often.
I believe Empire and MAC might have a written test, 50 questions from the ATP bank, and either a sim eval or a flight in the caravan as well. All of the only caravan operators hiring practices are pretty much the same.
Pay and benefits are also pretty much the same throughout the feeders, with slight differences. For example, Baron's pay is a little higher than MAC's Caravan pay but MAC has additional benefits like they'll pay up to $X a year for additional certificates or type ratings (I think $1,000-$2,000) you want and $x per year on school tuition. Baron's health insurance is 100% company paid if you're single, but costs significently more than Empire's if you have a family. They all spend about the same amount of $ on you, its just spread about differently.
If you get hired at WestAir you can "transfer" to one of the ATR or any other caravan operators later (say if you want to leave the west coast) with only a pro-forma hiring process and you do not have to go to flight safety again (unless you get hired ON the ATR) because all of the Caravan feeders operate exactly the same aircraft (same avionics) and I believe for the part 135 have identical opspecs selections. You can usually even keep your longevity... theres a pilot in ABI that worked for a few years at MAC in the east coast, but wanted to come back to TX and he started at Baron at the same year level pay he was at MAC.
The downside is that you will, baring a miracle, absolutely never get hired at FedEx mainline from a feeder. The only person I know who ever went over went because Fred Smith himself hired him after his Caravan had a mechanical problem, crashed and caught on fire and while injured he fought the fire with the onboard fire extinguisher and saved the freight and also the airframe, suffering second degree burns to do so. There might be a few others but as far as FedEx is concerned if they hire you not only do they have to pay your sim training at FedEx they also have to pay to train the replacement the feeder hires when you leave. It might cost a little bit more, but if FX ever instituted a "work at a feeder for 5 years and get an interview at mainline when we're hiring" rule the morale at all the feeders here would skyrocket to near deification of Fred Smith, not that the morale is bad here.... compared to a UPS feeder job this is a vacation.