How do I know the mark will come out?

Test on a scrap of the same fabric before you mark the quilt top, every time. Removal depends on the fiber and the finish rather than on the pen. A marker that vanishes from quilting cotton can set permanently on a coated or heavily blended fabric.

Water-soluble or heat-erase?

Water-soluble washes out and is the safer default. Heat-erase disappears under an iron, which is faster, but the mark can return in cold weather and some inks leave a shadow on dark fabric. Neither should be pressed before it has been removed.