No, they don't always have a marking on them. I have a pair of Het Pieds right now and the male is the only one with belly markings, or tracks as some call them. I got both from a friend that i trust and i have seen the parents and the whole breeding facility, no worries on my end. It's just something to help you out, but tracks or markings don't mean that it's a Het and it goes the same for snakes that look like complete normals.

The biggest thing you can do is be hopeful and if she turns out to be a normal, which could take a couple seasons, cause you might be all hets doing het to het pairing. You need to breed at least a couple times to see if you get a piebald, since you only have a 25% chance.