Hey John,

in general when you only have read permissions you won't be able to run any workflows at all. I'm not sure whats the intention of your question. To give a better answer to your question I'd like to understand what you want to do once you checked if a user has edit permissions.

Kind regards,

Enrico

I'm sorry, my original post was a little confusing.

Basically, I have a list with content approval enabled. Ordinary users with basic edit permissions can add items to the list, but moderators need to go in and approve them so the content can surface in other areas of the site collection. 

What would be nice is if moderators themselves (or those with elevated permissions, say like site owners) would be "auto-approved," saving them the inconvenience of having to go into the mod view of the list, and approving their own content.

From what I understand, there's no built-in way in SharePoint to do this. I thought I could accomplish this via a workflow, but getting the initiator's permissions and testing them is proving to be quite difficult, even with REST calls.

I think you do not need to check current permissions for that at all.

check rather WHO created the item.

set up a group of privileged users and check for their membership in group.

the most probably you already have such a group to grant permission on the list. so you could use the same if it's SP group.