With Bailey I was able to feel it on her back pretty easily, plus abscesses SMELL bad when they break or exposed to air.
With Rucy, I noticed her jaw on one side getting a little more swollen, and when I smelled her breath, it was foul - usually bunnies have great sweet breath! Anyway, to the touch, the abscess was a firm soft (if that makes sense) but not hard. When I touched it, it bothered her.
Now because her she has developed scar tissue from the surgeries, and also if an abscess happens and heals with medication, it can leave a hard harmless mass where it once was. So she has a funky jaw now - you can't see it, but I can feel it. Bumpy.
Because she's had an incisor and a few molars removed, some of her other molars can get spurs on them, and that can cause irritation. I don't feel any swelling, but I can see her eyes water more, and when she purrs, it hurts her and she jumps up. So I have to get her molar spurs trimmed about 4 times a year.