In the past decade or so, a lot of fake bronzes have turned up. They can be made very cheaply and enough people buy them from places like eBay that a profit can be made. A lot of fakes showed up years ago mixed with uncleaned coins, but on inspection these crude coins did not fool anyone. More deceptive coins turn up though, but generally there are things that are off with all of them. Overall, I don't think that fakes are very common though. One good thing also, once you learn to recognize some of the signs of fakes, you can spot them easier, for example- long split edges rather than proper flan cracks, these splits remind me of when you make cookies and press too hard, this is often indicative of pressed fakes. This is just one diagnostic, there are many ways that fakes differ depending on the method of manufacture. Plus, after you are familiar with some of the more common fakes, you will easily recognize them when they show up again.
The other issue is with cast coins. People counterfeited coins in antiquity cheaply and easily by making casts. It can be very hard and even impossible to tell an ancient cast from one made a few years ago.
Overall, fake LRB's are not a big concern; but I am sure that there are fakes so good that you would not be able to know the difference, but these are surely few and far between. If you are interested in how forgers make copies, I highly recommend the book Numismatic Forgery by Charles M. Larson, it is really informative.