Today’s vintage clothing analyst post digs deep to show you what era this dress is from and HOW WE KNOW.  Cheaters click here for the easy answer.
Let’s start with the things we know about it and can readily observe: 
1. Ruby red fabric
2. Shiny fabric
3. Buttons down the front
4. Cinched waist line
5. ¾ sleeves
Now, let’s look deeper for some hidden information (shown in the photos below):
1. Diamond armpit gussets
2. Rayon/Acetate fabric 
3. Center back zipper
4. Dolman sleeves
5. Self covered center buttons
6. Cotton shoulder pads 
In order to discover the era of a garment, look carefully at the silhouette. The cinched waist line with a fuller a-line skirt resembles the “New Look” by Christian Dior, instantly giving away the 1940s silhouette. But let’s pretend you didn’t know that, how could you tell from the seams what era this stunning ruby dress is from? Look closely to the sleeves, they have a looser fit known as dolman sleeves, and a diamond shaped seam known as the gusset.

While this does not automatically give away the era, look at the center back zipper on the dress. This is a fastener that started in the late 1940s and has been used onwards, so you can automatically cross off any eras prior to 1940s, but guarantee it is vintage. The self-covered buttons are down the center of this dress, setting it apart from the 1930s which had buttons down the side. 
Also, the shoulder pads are thick with cotton on this garment and sit on top of the shoulder. The cotton fiber and placement of the shoulder pad let’s you know it’s a ‘40s dress! The late ‘30s positioned pads to the right of the shoulder line, at the top of the sleeve. This is a subtle, but distinct difference to take note of. The front and back v-neck show the decade’s feminine open upper back neck line.

That amazing ruby colored fabric is made of Rayon / Acetate fabric, a well-known fabric used on dresses for the 1940s. It makes a good alternative for silk, making it nice to touch. However, it is a fragile fabric when wet, so gently hand-washing and air drying is the best way to keep this garment in mint condition! 
You have to pay close attention to detail when trying to discover a garment’s history, but that’s part of the fun! This dress has the sophisticated, gorgeous charm which defined the late 1940s after a time of despair and rationing. 
Disclaimer:  I’ve learned many tips and hints from various websites/resources and am not trying to copy/shortchange anyone’s openness to share what they know.  If you feel like I’ve gotten a fact wrong or haven’t given someone/a resource proper credit, please let me know.  Otherwise, the information I’ve stated here is to the best of my knowledge and learned from a combination of resources.  Thank you!