Is There A Book For Repro Roman?

The FORVM website has an excellent database of fake ancient coins: https://www.forumancientcoins.com/fakes/

If you know more or less what you have, eg. "Trajan denarius" or "Rhodes didrachm", you can search the database to try to find a die match. If you don't know what you have, post some pics and we can help narrow down the search for you.

Finding a match is proof of fakeness, but not finding a match is not necessarily proof of genuineness.

BOTH of Sap's recommendations are excellent.

Easy? Only if they are really obvious. There are basically 3 ways to detect counterfeits: 1.) by identifying a "twin" in a printed catalog or online, 2.) by understanding the characteristics of genuine coins and identifying outliers, 3.) understanding and identifying characteristics common to counterfeit coins.

One website with many images of fakes that hasn't been mentioned is https://www.forgerynetwork.com/

Regarding books, a number of volumes in the series 'Coin Collections and Coin Hoards from Bulgaria' (CCCHBulg) deal with ancient coin counterfeiting, both catalogues of fakes, and discussions of counterfeiting methodology and resulting characteristics. Most of these volumes have been made available as free pdfs by Dr. Ilya Prokopov via his academia.edu page. There are many titles listed. You'll have to scroll through and pick out those of interest.

https://independent.academia.edu/IlyaProkopov