I look at the horn from a few feet away from a few different angles, and I also look down the bore. Usually I will put a leak light in the bell to illuminate the bottom bow and make it much easier to see. I also use my fingers on the body tube to feel for twists or ripples in the metal.
Personally I like my horns straight whether or not the bend is causing leaks, but then again I don't usually charge myself too much. The main problems a body bend is going to cause are:
1. Key fit problems. Posts could either be too close together (perhaps on the concave side) or too far apart. Whatever caused the body to bend was probably somewhat major trauma, and the weight of the keys on the body and their inertia during said trauma can be enough to move posts that are not even in the area of the bend.
2. Leaks. Either from the newly introduced play, or the key is sitting at a different angle relative to the tonehole than before.
3. Bell angle. If the body is bent, check the seal on the bell keys. Bells can move relative to the body, and often move a little during whatever happens to bend a body. Depending on the design of the horn, this can introduce different issues. For example on horns that have the bell keys on posts on the body (like a VI) the angle of the keycups can change and introduce leaks. For horns that have multi-part bell keys (like a Dolnet or a Conn Chu) the pads will probably still seal but the LH pinky table may no longer be in adjustment.
I've seen and played a lot of horns with bends in the body, and many sax owners are surprised when I point out a body bend to them when they bring in a horn for another reason- so it is not always something that affects playability.