It is easy to tell when you have actually seen some in the past. The rule of thumb is when "it looks too good to be true" although it may not be as useful to paper money of common dates, denominations and/or varieties.
What helpful tip I can really tell you is this: fake paper money usually feel just like an old piece of bond paper, one that has already toned reddish-brown through the years. If in EF-UNC condition, it will be easy to tell they're fake just by flapping them in the air, because they will not sound as crisp as a real one. Experiment with your newer bills to familiarize yourself to the sound of real ones in these grades. But in VG-VF conditions, it's not gonna be easy to tell a fake one from a real one. Most fake bills I've seen come in these grades, usually at around Good to Fine.
I've seen some fake American and Canadian bills in the past, so I pretty much know the tricks of the trade. Sadly, it's not as easy and as straight-forward as some might think. You have to take many things into consideration, like the material/s used for certain bills from certain time periods. Take the Japanese Puppet Money for example, they all feel like old bond paper. So how would you know if yours is real or fake? I guess they're all fake money to begin with anyway. But you get my point. LOL.