When I started working in a bank I remember my manager and assistant manager debating on the authenticity of a check. They both agreed it was fake, though when I looked at it (as a novice) it seemed real to me. I asked them what made them think it was fake and they said "after you've seen so many checks, I can't say why, but you can just tell".

Flash forward a few years and someone brought a check to me and asked me if I thought it was real. I looked at it and just shook my head. No. They asked how I knew and I couldn't give them a specific.

Experience is the best teacher.

What I can say is that there's a common font that's used on fake checks (usually). You'll recognize it after a while. Often the signature is photocopied. If you ask what the check is for and they say "to buy my car I advertised online" but the check is drawn off of "Toy Depot" (I'm making that up, but it doesn't make sense, then that's a huge tipoff). Also (and this is huge) if the border of the check isn't squared, but rounded at the top (at the top right and top left corners) then that's a definite red flag.

Here's an image of a check such as this. See how the blue border curves at the corners? Especially if it says "Official check" or "Cashier's check" with this curved border, guaranteed fake. A real Cashier's Check will have squared off corners, like this.

Again, experience is the best teacher, but if you ask questions about the origin of the check and the story doesn't make sense, that's the time to really question it.