how can I tell what my bike is worth
You could google the model name and see when it was introduced.
You can check the components models (brakes, shifters, derailleurs...) to see when they were introduced.
That should give you a starting point.
You also could try contacting Specialized.
Although it is difficult to read the type on your slightly fuzzy picture, several details suggest that it's an enduro or downhill type Specialized: massive frame, fairly long front travel, big brake discs, rear triangle that allows big swings.
Searching for Specialized and enduro produced a nice survey suggesting that it dates from 2005-2006. https://www.pinkbike.com/news/the-evolution-of-the-specialized-enduro-2016.html
There are a few ways:
Google the brand and model and guesses at the year, select 'images'. Trawl through the images until you find one that matches your bike.
Go the Bicycle Blue Book, look up brand/model, guess year, trawl through images and specs until you find one that matches. That will also get you an idea of what you can sell the bike for.