I live in Sweden so I know exactly the type of rowan you're talking about. I actually took one down this weekend. It can be absolutely straight and still be twisted wildly underneath the bark. I haven't found a way to tell the grain from looking at the bark. It often also varies in a trunk – the grain can be straight for a couple of feet, twist some, then straight again, then twist the other way... What I do is I carefully cut the bark off in small patches and look at the actual wood. You can see the fibers if you look closely. If the first patch reveal twisted grain, move along. If it's straight, cut another patch perhaps a couple of feet from the first and have a look. Repeat till you've made sure the entire thing has straight grain.
Rowan is strong in tension, it's weakness is compression. So take you time to dry it well, then it'll be stronger in compression. I think it would work well with a MC of about 7 %. It does sometimes crack on me when drying it too fast in the beginning, so cut/split it as thin as possible (1-1,5" or so, depending of what kind of bow you're hoping to make), debark it, glue the ends, and let it sit under roof outdoors or in cool basement or such for a month, then you can take it indoors.
