Simple. I evaluate third party support by the level of commitment I see from the big players, and I measure that by the amount of flagship-caliber projects targeting the system (either as the lead platform or as an exclusive). Are the big guys putting their best teams on Switch projects that aim to take advantage of the system's features and/or audience? Right now, only Capcom (Monster Hunter Rise, Monster Hunter Stories 2, the potential games leaked through the hack), Koei-Tecmo (lots of stuff) and Atlus (Shin Megami Tensei V) fit the bill. I wasn't expecting much from the big Western publishers in the first place, but the answer is 'no' even from many big Japanese publishers.
The Switch doesn't have a focus on a touch screen, motion controls or asymmetrical gameplay. Aside from its hybrid nature, it's just a standard console with a standard set of controls, but with an audience that does not put as much importance on cutting-edge graphics, movie-quality cutscenes, etc, and that is buying a variety of software in droves worldwide. Normally, developers would see this as a big opportunity, yet;
Sega (I see Atlus as operating independently from its parent company) are sitting on a pile of IP that could use a revival without the budget needed for it to stand out on the other consoles. Phantasy Star, Jet Set Radio, Skies of Arcadia... They could have brought back Sakura Wars as a strategy game leverages the Fire Emblem audience, yet they chose to make it into a generic action RPG and only release it on PS4.
Square-Enix could make a timed exclusive FF remake or a brand new mainline FF that is more of a throwback. How about a new FFT or a new Chrono game? Maybe a new IP with a decent budget behind it and whose name doesn't sound like a working title? Nope. What they *are* making, however, is 'Dragon Quest for Adults', which is basically code for targeting the Western PC/PS/XB demographic.
Namco are mostly doing late ports. Smash Bros clones are all the rage now, yet all we get is Jump Force instead of a new Jump Stars-style game. They sure as hell ain't making Tales games for the system, or else this forum wouldn't exist. In fact, I would say that the series of events that led to the creation of the two new forums is the result of Switch third-party support not being so unquestionably good, all things considered.
The SNES and DS may not have as much third party software as the Switch, but there was never a sense that developers were not trying their best on those platforms. The former is home to some of that generation's defining third party games, and the latter got at least 5 good years of exclusive, high-quality software that was often tailor-made for the hardware. As of right now, I cannot see either of those happening on Switch, hence me rating its third party support lower than those other two platforms.