Cock, load, close, put a tissue over the breech and fire in a safe direction. If the tissue blows off or or gets a hole blown through it, you have a bad breech seal.
You could also check for built up grease/gunk around the barrel locking wedge. Too much build up throws off how the barrel/breech closes and can throw off accuracy.
Stuff I've mentioned is basic to all guns. Honestly figured someone who's actually owned your rifle would have chimed in by now with more indepth ideas about that exact rifle since the only one I've ever had was an older Bull Whisper I got cheap from a yard sale that ended up having a barrel so bad pellets just fell completely through it.