iNaturalist will often show both of these as possible IDs, and when I look at them side-by-side, I have a hard time seeing the difference. Can someone tell me what to look for to distinguish between the two?
Welcome to the Forum!
Are these the specific plants that you are referring to (I was not sure if there are other plants with the same common names):
They may help you with identifying the differences.
I do not know much about California plants or Irises but I do look forward to hearing what someone that is familiar with these plants will say.
And if you do not get a response here, you can also message one of the Top Identifiers of a particular plant on iNat to see if they could offer help. For example, you can find the Top Identifiers here for Iris macrosiphon (Bowltube Iris) or the Top Identifiers here for Iris douglasiana (Douglas Iris).
I’m glad you asked the question, because those look alike to me, too. The location also helps. Here is a range map for Douglas’ Iris (Iris douglasiana): https://www.calflora.org/app/taxon?crn=4346 and here is a range map for Bowltube Iris (Iris macrosiphon): https://www.calflora.org/app/taxon?crn=4355. So if the location is away from the coast, Douglas’ Iris seems less likely.
This page may help, in that it both says what the differences are, and has diagrams of the parts referred to:
In short, it comes down to how long the tube between the ovary and petals is.