Having seen so many union flags done wrong this weekend, here is my simple guide of how to tell a good flag from a bad one. It is not hard to tell once you know, and it is a curse, just like learning correct grammar is a curse.
One issue with spotting a duff flag is that you do not always see all of the flag. Some times you have a stylised design showing part of a flag. Even so, there are clues.
I am not going to try and mention colours (yes, the blue is meant to be quite dark compared to the red), ratios (as the design rules allow for different ratios, and there are several in use, though 2:1 seems "normal"), or try and cover every way it can be wrong, just the main ones. These are all things you can see without needing a ruler (no, I do not mean without a King/Queen, I mean without measuring things).
1. Right way up
The red diagonal cross is not the same as the original St. Patrick flag but sort of shifted anti-clockwise slightly. It means that the red diagonal cross is lower on the left side than the right when viewed with flag pole on the left. When no flag pole you assume it is on the left. Otherwise it is upside down (supposedly a sign of duress).
2. Red at the corner
At each corner one edge the St. Patrick red cross touches the corner exactly. It is either the top or bottom edge depending on the corner (see point 1 above). It is not centred on the corner or missing it at all.
3. White at the corners
The white St. Andrew cross on blue does meet at the corners and is centred on the diagonal. In fact, the white:red:white ratios are 1:2:3 and the total thickness of the white cross is 1/5th of the overall height of the flag (the same as the thickness of the red part of the St. Georges red cross in the centre).
4. Red diagonals meeting the centre
You cannot always see the corners, e.g. when the flag is on triangular pennant or some other stylised version. You can, however, spot simple mistakes. The diagonal lines of the St. Patrick's cross meet the red St. George's cross in different ways depending on the ratio of the flag, but the top and bottom diagonals do not meet in the same way - in fact the top left diagonal must have its right hand edge aligned with the left hand edge of the bottom left diagonal where it meets the centre. i.e.