For bills that are $10 and higher, the number in the bottom right corner will appear to change colors if you look at it from a different angle. Counterfeiters will sometimes try to replicate these by printing red and blue lines onto the bills, so compare the texture against another bill - preferably from a bank.
Genuine bills will have red and blue threads woven into them. Examine it more closely for further signs of counterfeiting. If you touch a bill and it doesn’t feel right or familiar, it might be a counterfeit. US money has a number of identifying characteristics you can use to check if a bill is genuine. All international money transfer services.
