Engineer Rob Wilson was helping us test one of our upcoming puzzles, and as we debated whether a particular piece was part of the straight edge or not, he shared this great tip:
"If you run your thumb along the edge, you can tell if it's truly straight."
We tested his tip on a few pieces, and we agree, it works!
That said, we do sometimes deviously include some false edge pieces inside the puzzle that are perfectly straight but not actually edge pieces, and if you've got an irregular edge puzzle, well then this tip isn't going to do you any good at all :).
And here's a link to our straight edge puzzles.