Summarize:

Often a rat will scratch a flea infested part of its body more frequently and often quite vigorously. This area can become irritated, red, or inflamed from scratching and from fleas feeding on your rat's skin. Missing patches of fur or red scabs is a good indication that something is irritating your rat's skin even if you don't see them scratching. Fleas like to hide in densely covered areas of animals. If your rat is scratching its armpits and belly a lot more than normal or has irritation in these areas, it might be a sign of fleas or some other skin irritant. Your rat might jump or react violently out of the blue. This is a sign of hypersensitivity to flea bites. If the infestation is heavy, you will be able to spot fleas or its feces. Flea feces or flea dirt will look like coffee grounds on your rat's fur. Use a flea comb or a comb with closely set teeth and run it through your rat's fur. Make sure to run it close to your rat's skin in order to catch and pull any fleas that are hiding under the fur coat. If your comb comes up with any black spots that are dirt-like, it may be fleas or its feces. Have a bowl of soapy water on hand to wash your comb of any fleas you might pull out. Flea feces and dirt can look quite similar, especially if you bring your rat outside to play. One way to tell the difference between the two is to wet any dirt-like specks with water and watch if the spots turn a reddish-brown colour. The red colour is the digested blood that the fleas have ingested which has been broken down by the water. Adult fleas account for about 5% of a flea colony or infestation. The other 95% consist of flea eggs, larvae, and pupae which are harder to see with the naked eye. Check any areas your rat frequents or sleeps, or even areas near or under its cage. You might be able to see flea dirt or fleas in your rat's cage or bedding, especially if the flea infestation is quite heavy.
Observe if your rat is scratching more than normal. Look for fleas in your rat's skin and fur. Look for fleas in your rat's bedding.