picture of a rat Wikipedia
The biggest tell is that they can get pretty big. A huge (larger than your hand) rodent that doesn’t look very overweight will almost always be a rat. The overweight ones will probably be rats too, people tend to overfeed their rats. Their face shape tends to be more square and blunt than in mice, especially when you look at them from the side.

picture of a mouse There are a lot of very different Mice species (especially when looking at wild mice) but if you had to know just one it should be the Fancy Mouse. It’s not really a species, just the term for mice that have been selectively bred for domestication, so they can look vastly different in color and fur texture, but the overall body shape is somewhat consistent.
Generally, mice stay fairly small (generally smaller than your hand) and their head to body ratio is somewhat different compared to rats (I’m actually not sure what exactly looks different, maybe less neck? It just looks different when you look at them side by side)
Their ears will also be bigger than rat ears (though large-eared “Dumbo” variations are a thing for both mice and Rats): Picture 1 is a Dumbo Mouse, Picture 2 is a (young) dumbo rat:
Dumbo mouseDumbo rat

There are two variations of hamster you should generally know: the larger Syrian Hamster or Golden Hamster and the Dwarf hamster , which isn’t a species but more of a catch-all term. In captivity, a Dwarf hamster is usually going to be a Campbell, Roborovski or Djungarian Hamster (or a mix thereof).
Syrian hamster
Syrian hamsters are larger than dwarf hamsters and Mice, but usually smaller than rats (some very fat hamsters exist of course). They only have a very short stubby tail and a round face. Very round if their cheek pouches are full.
Syrian hamsters are strictly solitary and extremely territorial, you should never keep more than one in one cage (optimally, you wouldn’t even have more than one in your house). They will kill and eat each other on sight. Don’t let anyone tell you that “it works for their hamsters”, it does not. Dwarf hamsters
Dwarf hamsters are way smaller than Syrian hamsters and about as large as a mouse (depending on the species ofc). They’re pretty recognizable because they’re very round. Usually they don’t have a tail either, with some species being an exception and having a slightly longer stub.
The biggest difference to Syrian hamsters is that Dwarf hamsters are very social!! Generally, you should never keep them alone, as they bond to each other and build social hierarchies. Solitary Dwarf hamsters can die from the stress of loneliness. It’s sometimes even recommended to keep your groups number even so that so hamster is left out when every one else bonds as a pair.
Keeping a solitary Dwarf hamster is less dangerous than keeping a pair of Syrian hamster though. Some people say that they can make it work by interacting a lot with the dwarf hamster so that they don’t get lonely. I don’t really know if that’s scientifically proven though.

Here are some more common pet rodents you might want to know about:
Chinchilla
Chinchillas: Big boys, need a lot of space to run around and sand to bathe in.
Gerbil
Gerbils: About as large as Syrian Hamsters but usually pretty lean looking. You can recognize them from the fact that their tail is covered in fur. Guinea Pig
Guinea Pig: You might confuse them with hamsters from a photo, but they are way way larger and more square, especially in their face scape. They’re also pretty vocal, so you’ll hear them coming lol.

Here’s images of rodents, what are they? image 1
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image 10

image 1: Rat
image 2: Beaver
image 3: Syrian hamster
image 4: Fancy Mouse
image 5: Fancy Mice image 6: Trick question, they’re African Soft Furred Rats, a species of mouse
image 7: Guinea Pig
image 8: Rat
image 9: Dwarf Hamsters (Roborovski, I think?)
image 10: A young Dumbo Rat