But I don't know what the wards are. And if they don't work... how can I know if I've run into one??

A near perfect ward would be to place 12 thumbtacks or something similar on your door.
Then try and spend time in your room. If it anything unusual occurs eg. Stress, fear of the unknown even physical or mental pain, you will have a fair indication.
Also if you see another Shadow and feel connected to it like you would with a close human relative that will also help.
It is hard to find a clear distinction between Shadow to Shadow as when they drain human there becomes an emotional attatchment of sorts.
Hope that helps.