Just wanted to chime in on this, with some things I've come across during my research into this:
Another quick way to differentiate between the 2017 v1 "Erista/T210" model and the 2019 v2 "Mariko/T210B" (especially when looking at stock in a brick-and-mortar store), is to look at the box itself (if you can see it, that is, such as in the glass case at a Walmart).
The packaging for the Mariko units has a solid red background, with the Switch seated in the dock, the JoyCons attached to the included frame, and a TV behind them.
The v1's packaging features the same Switch (docked) with a person's hand on it, the JoyCon, the TV, but a solid light greyscale background (you could even say it's white).
The rest of the box art is nearly indistinguishable between the two. See the pictures below.
original v1 box design:
new v2 box design:
In the case that it's a used/refurbished or new, unboxed unit, a quick glance at the model number on the back will also tell you which hardware revision it is. The v2 one is
HAC-001 (-01), while the v1 is simply
HAC-001.
Another difference to look for, in the case of an online store, is the model codes. Sometimes the retailer will simply state "Nintendo Switch v2," "new model," or "improved " Sometimes they even include the model codes.
The new Nintendo Switch has a model code that starts with
HAD, followed by a "color code" for the included JoyCons:
HADSKABAA for the blue & red Joy-Cons.
HADSKAAAA for the grey Joy-Cons.
The v1 model's code starts with
HAC.
Of course, once you can visually determine if it's a v1 using these methods, take a quick moment if possible to look up the serial on the site
@dangopig linked above. Otherwise, keep searching elsewhere. Good luck!