It has been passed many years since the last time I played to Wolfenstein 3D. Is there a visual trick to detect secret area or should I try to press open on all walls?
Well there are secret rooms and secret levels...
Although there are no strict guidelines to where a secret room might be, there are subtle hints that might help you find them. It helps to map out the level since the layout of the levels are accurate (no magical warped spaces), gaps between areas could hint at secret rooms.
As for secret room markers, first try the obvious places on the walls (e.g. Hitler paintings, Nazi banners, and other wall decorations).
Some secret panels seem to be in the corners of a room, while others can be found in the middle(point) of a wall (usually in a room).
Also, be on the look out objects placed against walls between sets of twos (such as plants or barrels).
Some secret panels can also be found by walking in a straight line from (either side) a object (such as an overhead light), or in narrow alcoves.
Take note that certain secret rooms have secret rooms within themselves(!!) and sometimes the movable panels of secret rooms can block off access to other secret rooms (this why you sometimes don't a get 100% secrets even though you've searched every nook and cranny).
In every Episode, there is a secret (10th) level you can access by taking a secret elevator. There are pretty hard to find if you don't have a map to help you. It is usually accessible though a series of secret doors/panels within the secret rooms and usually located in the last room of that level (the room with the regular elevator).
Here is a list of what levels the secrets levels are on by Episode.
Here is a list of all the secrets in the PC version.
Also everything you would typically want to know about Wolfenstein 3D.
There is no visual trick to detect secret doors. The only way to find them is to try and "open" the wall as if it was a door. Sometimes the secret door is in some place which is stands out a bit (e.g. a wall painting, an alcove, the center of a wall etc.) but many times there's simply no indication at all.
The best way to find a secret door is to run diagonally into the wall - so that you stay touching the wall but constantly moving - and click the open key repeatedly.
However, keep in mind that:
No, there isn't.
I spent hours in 1992 staring at walls and listening to BJ going "uff ... uff ... uff" as a tapped the 'open' key. Now you can, too!
Usually they will not be unmarked along a flat wall, but they could be in a alcove (marked or unmarked) or be represented by a painting (or if I remeber right, maybe a suit of armor) on a flat wall.