How can I be sure I blew up my WS2812B strip?
DIN is not connected through the entire strip, so that is fine.

I'd just test by giving 5V power to the entire strip and checking that there is only a negligible current flowing (so none of the LEDs are shorting the supply), then giving it a data signal it can understand and seeing whether anything lights up.

If it doesn't work feeding data to the first LED, try the second (because the second LED is fed data from the first, so if the first one is dead, the rest of the strip won't light up either).

Test your data signal against a good strip so you see how it should look like -- and connect the data bit after the power, because DIN at a higher voltage than VCC is also an easy way to fry the LEDs.