At present I'm not aware of any widely-deployed OS where an app can read the received ECN markings. 

iOS9 added support for this within the kernel, and it's used for TCP, but not exposed to userspace. There is an open Radar bug asking for this info to be exposed to userspace.

FWIW, Chrome supports setting the DSCP markings if you set a magic parameter. But it's not on by default, mainly because we've never done the auditing necessary to ensure this doesn't randomly break in various dimly-lit parts of the internet.