Steering alignment should be done once a year or when excessive tyre wear is noticed. The front wheels should ‘toe in’ between zero and 1/16" (0-2mm). That is, the measurement across the front of the wheels should be between 0 and 1/16’ less than the measurement across the back of the wheels, at axle height.
A check can be made with a tape measure, and adjustment made by undoing one of the steering connections (eg steering arm on kingpin) and screwing one of the rod ends (ball joints) in or out half a turn at a time. Note you will notice that the rod ends have a jam nut (lock nut) to prevent them unscrewing and to eliminate movement on the thread. This will have to be loosened first before the rod end will turn and locked up again before each measurement is made.
A more accurate way is to use a trammel (see fig. 15). A trammel has two pointers on a bar, which sits at the back of the tyres at axle height. A mark is made on the tyre with a pen, the trike rolled forward half a wheel revolution, and the marks compared against the trammel pointers. This method eliminates any inaccuracies due to wheel run out etc. All measurement should be done with rider on seat if possible as this helps flex and preload everything into position.