Ayup mate,
First off you can take all the precautions you want but some times it's just impossible to tell if its bow wood until it's down, split and debarked.
What you are looking for is no knots, straight as possible with no twist. With ash you want fast grown with vthe little porous early wood and nice thick late wood rings. This can be guessed at by where the tree is growing and what trees have grown around it. If it's been starved of light growing under a load of pine or spruce then it will more than likely have a poor early to late ratio.
Problem then is if it's grown in the open it tends to have a lot of side branches.. so your looking for something in the middle.
It's is sometimes possible to look closely at the bark to see if there is any prop twist but you can also feel for the overall cross section which is sometimes and oval shape this will give you another pointer.
If you see signs of prop twist just walk away.
Size wise it's just a case if what you can deal with in terms of weight tools and permission. Also before dropping the tree make a note on the bark with a pencil or marker which side is in tension and which compression. Saves trying to remeber later.
So considering all of the above if we find the tree we are looking for the next very important step is to seal the stave properly after splitting. At this time of year the ends of the stave can start splitting whilst you watch them. Besides sealing the ends with pva glue make sure you leave plenty of length so 80 inch if possible as this allows for longbow lengths minus a few inch.
More later when I think a little more about it. ;-)
