^^^^
There used to an overhead sign on I-66 eastbound west of Cedar Lane noting the truck ban, but as you note the construction caused its removal (see below). The overhead signs on the Beltway used to say "NO TRUCKS" as well on the changeable portion, but those got removed when the HO/T system took effect inside the Beltway. VDOT has always seemed far more interested in signing the HOV restrictions than the truck restriction. The exit signs when you reach the Beltway really ought to have an "ALL TRUCKS" message on them (or at least, the second right-side ramp leading to the Inner Loop should have such a banner, as that's the last point at which large trucks are allowed).
https://goo.gl/maps/TM215i1VigrbC6q79