I am looking at mid-40s vintage model 70 that with blueing that just looks a bit too vibrant and the bolt shroud is a bit purpled. Is there a sure-fire (and quick/easy) way to tell if something has been re-blued? TIA.

Start by looking at the lettering, reblued will usually be washed out or rounded of..serial numbers also. Look for past pitting that may have been buffed out..swirl marks, etc.,

Not trying to be a wise ass but if it is a good blue job who cares?
As a collector of firearms in America that seems to be one of the big questions and no no's! In England many of the fine English guns were sent back to the factory at the end of the hunting season for cleaning, repair if necessary and often re-blacking. Nobody seems to care about this if it is collectable English gun. Only Americans seem to want the original rust and dust.
When looking to see about a re-blue I try and mirror the light on the metal. If the reflection of the light is straight on the metal then it is either the work of a true craftsman that re-blued it or the original finish. Buffing a bbl. wears off lettering as described in other post and often will cause ripples in the longer lenght of the metal and that will show up as a crooked light reflection. Shows somebody did not know their business.
Then you can always put it up against a pc. that you know to be true and compare.
The bad lettering could sometimes be at the end of life for a factory roll mark die too.
Strange things happen. Trademark Texan bought a new Mod. 70 off me that had NO caliber marking on the bbl. The SN on the end flap on the box and the SN on the rifle were alike and correct. The box showed the caliber but the bbl. was not marked as to caliber. New in box rifle.

Most pre-war firearms have a dull black finish that is often referred to as charcoal or carbona bluing. The high luster bluing came into popularity in the post war years.

As stated above, a re-blue doesn't necessarily deminish a firearm, especially if the original formula was used. The question you have to ask is whether it was refinished to preserve the metal, or to hide abuse. Look for signs of buffing, as well as for scratches that contain bluing.

Many original rifles will have rust blued barrels, but hot blued receivers. Different metallurgy will also cause some parts to look purplish rather than black, right from the factory.

I automatically assume that any firearm that is not NIB has the possibility of having been refinished at some point in time, especially if it was made over 50 years ago. What I look closely for is whether the metal is being preserved in its original form, or has the metal been buffed and ground to hide abuse or neglect.