Purchase an American Standard of Perfection to learn your breed Standard and how to evaluate all birds (many people think it is worthless to buy and SOP for just one standard, but it actually has a lot of very useful general info for all breeds in the first 50 pages that is just as important as the breed standard, telling you how to select a healthy, productive bird and all the general defects and disqualifications.)
You can order the SOP on the APA website (or used, but if anyone charges more than 70 dollars for it they are scamming you.)
Right now, the 55th edition is under revision, but for the meantime, you can borrow the SOP from the library and you can make photocopies of the useful pages.
The Rhode Island Red Club has a website with a gallery that shows exactly what very good specimens are supposed to look like.
https://rirca.poultrysites.com/index.php/photos
If you are serious about RIR I would recommend joining the club and if you have Facebook, join their Facebook group.
Anyway, to start with I will tell you how I interpret the Standard of the Rhode Island Red, but be sure to refer to the actual Standard rather than my paraphrase.
All this information I credit to the American Standard of Perfection:
The Rhode Island Red is to be a long, rectangular bird (many people describe them as brick-shaped) with a low tail, well spread tail, 20⁰ above horizontal on males, 10⁰ on females. The back transitions smoothly to the tail. Large wings carried horizontally. Cock 8.5 lbs, Hen 6.5, Cockerel 7.5, Pullet 5.5
Birds are to be a lustrous, rich dark red right down to the fluff. The tail feathers are to be lustrous greenish black on the male, except for the sickles which approach the saddle. The main tail feathers (the 14ish long ones) on the hen are to be lustrous greenish black but the top two may be edged with red. Hens will have black tips on the end of the lowe neck feathers. Primary wing feathers should have a clean black lower edge. There should be no mossiness (meaning black flecking) anywhere in the plumage.