Measuring chainline without cranks?
You can measure the chainline at the cassette as well as at the cranks.

Park tool Company has a nice article on measuring chainline at both the crank and cassette.

https://www.parktool.com/blog/repair-help/chainline-concepts#article-section-3

It's my understanding that bikes sometimes have the chain further offset from the  frame centerline at the crank than at the cassette, to avoid the chain rubbing on the next bigger ring when small ring - small sprocket cross-chaining (triples especially).