"...In the absence of a proper survey having been completed I would stake a series of contours over the site and paint-mark (or ribbon) the stakes different colours that alternate from one contour to the next (2 colours should do) so that you can differentiate between each marked contour.
Then start marking out the guideline offsets again using alternate colours (though different to the contour colours) and then compare these offsets to the contours. You're doing here what we'd normally do on the screen following a detailed contour map.
If the guideline offsets are going where you want them to then go back to the guideline and adjust it and try again (and again!!) until you're satisfied..."