a better idea than looking for signs of founder, is preventing ponies (and horses) from getting excessively fat in the first place. As soon as our ponies start getting cresty and fat deposits on either side of the spine, we restrict their grazing. The old saying 'prevention is better than cure' is just so true in the case of founder.
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare