Index cards can be useful for this. You can either allocate a symbol for each thread, or choose a colour, and then go through scene by scene marking where the threads appear. You see very quickly if there are any holes, and where you need to add a mention of a particular thread to keep it going in the reader's mind.
Another thing to check is how quickly a thread appears, and if it gets set up properly. For example, towards the end of Adultery for Beginners I realised I was using colour to show how Isabel was changing and asserting her own independence when she chooses to paint the walls in the house strong yellows, blues and reds instead of magnolia. I added a scene at the beginning where Neil and Isabel go to choose a sofa. The main purpose of the scene is for them to discuss her job offer, but I added that Isabel wants to have an adventurous colour whereas Neil goes for a dull brown. I also added that Neil is keen on texture, and wants a luxurious chenille - a hint of his interior character which will also emerge later.
Sometimes threads are about plot, sometimes about characters, sometimes they'll be images o symbols. All need to be woven in carefully to make a cohesive whole.