The newly formed Finch Research Network put out a really great post on redpoll ID. Lots of pictures, lots of ID tips, and some fascinating discussion of some of the finer plumage features.

I've been going back to this page multiple times this season:
https://finchnetwork.org/north-american-redpolls

They also have a nice, succinct summary of the most important ID features. Having spent some time looking over redpoll flocks this winter, it can be really challenging to get definitive looks at these features. Distinguishing these two species is a true ID challenge, so don't be afraid to say "I don't know."

Good birding,
Allan