Accurately, you'll have to go to a sport's medicine place and get some tests, but that costs money and that level of accuracy is unnecessary for deciding to bulk or cut.

If you want a ballpark, there are a few different methods. There's comparing yourself to pictures: http://www.builtlean.com/2012/09/24/body-fat-percentage-men-women/

There's bodyfat calipers: http://www.amazon.com/gp/product/B00B2MIDFY/ref=wms_ohs_product?ie=UTF8&psc=1 This didn't work out for me because I'm squeamish and hate pinching my skin under fat and stuff.

There's impedance meters: http://www.amazon.com/gp/product/B000FYZMYK/ref=wms_ohs_product?ie=UTF8&psc=1 The theory is the more fat you have, the more a current is impeded as it flows through your body.

Good job. Next step should be to maintain this state for the rest of your life.

To estimate your body fat percentage, you can try one of these calipers for $14: https://www.amazon.com/gp/product/B00B2MIDFY

If I had to guess, I'd say you're below 15%. But if you worked this hard to lose 100 pounds, I think it's worth spending $14 to get a better estimate.