It is not very difficult to distinguish spoiled or bad foods, look for the following sings. Bad taste with as small sample, use as a last resort. Dip your fish into the batter and when placing it into the oil, don't just drop it in. Hold the piece of fish as close to being submerged as possible, until it wants to float on its own, then release it so that it does not sink to the bottom and get stuck. So the more humid the conditions are the more damp the flour will be and the more flour will have to be added.
Recipes rely on the experience of the chef to know when enough flour has been added.
You will have to look for certain signs such as consistency to know when enough flour has been added. Slicing the roast against the grain will help to promote tenderness. Slice the roast in half and look for the directions of the grains. If you are already going against the grain you will see diamond shaped holes on the exposed sides of the roast. If not you will see the grains running along the sliced sides, slice accordingly.