We all know some common symbolism: For instance, red stands for anger, blood, and violence. The cross stands for martyrdom and sacrifice. Common symbols are easy to notice, but what if you decided to use uncommon symbolism, how do you cue in your readers that you are using a symbolism for a meaningful reason? Is there a way to do this subtly? Is there a way to do this at all? What do you think?
One way to do it subtly might be to show the character's reactions to the symbols. For example:
John was about to enter the room when he noticed the tulip painted to the side of the door. He hesitated. Not that he was superstitious, rationally he certainly knew that this was just the image of a nice flower. But that couldn't convince his emotions. After all, more than enough bad things had happened to him this day already.
Anyway, his rational mind won, and he entered the room, although not without looking extra carefully at every corner where someone could hide, despite his analytical mind telling him that there should be no danger at all. And indeed, he reached the other end of the room without any incident.
He relieved, although he hated himself for it. It was just as stupid to consider himself safer now as it had been stupid to worry because of such a stupid symbol, and he had lost valuable time. He swore to himself to ignore those symbols from now on, and firmly stepped through the next door.
And that was when the shot hit him.
Note that nowhere I told what the symbol means. Yet I'm sure you have at least a rough idea about its meaning by now.
Depends on cultures too - red also means prosperity.
If you wish to use somewhat obtuse symbols, use them. Do not then say to the reader ‘I know you will never get the allusion on page 142 so I will explain it to you”.
Some symbols add dimensions of meaning that subtly influence and improve the whole - like using bacon fat to sear the meat in a stew - adds something appreciated but not necessarily identified.
I like novels that reveal different layers of meaning, different shades of characterization and details missed on the first few reads.
Add when appropriate, like seasoning your food.
Remember that the reader is an intelligent person who has read many books before. If you treat them like children, they will perceive this disrespect and probably put your book down.
You may want to consider how necessary the symbolism is to your story.
If it's something that's important for the reader to understand in order for the rest of your story to make sense, you should probably have some explanation. An Example could go like this:
As he stared up at the swirling patterns on the guild hall office emblem Repomonster was reminded of his childhood dream to join the guild. He knew what each swirl represented...
Another example could be using an ignorant character and having the symbolism explained to them.
On the other hand, if the symbolism is not critical to the reader understanding the rest of your story it may not be worth taking the reader on a side trip away from your plot to explain. Leave it there as an Easter egg of sorts for attentive readers to find and appreciate.