Is there a way to test a horse in Steeplechasing without making him/her a Steeplechase horse forever?

At the moment, no.  I think general rule of thumb is that if the horse prefers turf, he’ll probably do well at steeplechase.  The steeplechase circuit isn’t as big as those for the flat, so unless your horse is consistently failing to bring home a paycheck it might be better just to keep to flat because it’s so much harder to find races for steeplechasers.

Certain lines, such as Lonesome Glory, also tend to throw steeplechasers.

What other lines tend to throw steeplechaser?

Highland Rogue tends to be jumper-y.