If she has very pronounced wrinkles either side of her head she`s probably getting ready to shed, my larger snakes do that often and it`s nothing to be concerned about. I don`t keep Dummerills myself so check what I`m about to say against the species care sheet...
Usually, when snakes are about to shed, it is good practice to either up the humidity a little with daily spraying (don`t soak the tank, just a light spray, a hygrometer wouldn`t go amiss) or to add a moss hide. A nice plastic tub JUST big enough for the snake to squeeze itself into, with a good inch of very damp but not soaking moss in it. This helps them shed thier skin. Sorry if you already know all this and I`m teaching you to suck eggs here lol I expect if she has wrinkles, her head will be looking a little dry aswell?
As for telling if she`s under weight, a few good pictures will be good, I`m sure we`ll be able to tell if she`s grossly under weight. Short of that, have a look at her back bone, or more to the point, can you see it? If a snake looks almost triangular in cross section that`s not a good sign, you really shouldn`t be able to see a pronounced back bone in a heavy bodied species like the Dummerills Boa. I`d say she wants to look good and rounded, heavy set bright eyed and alert. Have you searched for images of Dummerills Boas on the net? Look at enough pictures and you may build up an idea of what they should look like at her size. I suppose your best bet though is another Dummerills Boa owner to tell you what they should weigh at her size. Hope that was at least a bit helpful, I`m sure every one else will have lots to say soon enough....