I use a hose level. To make one obtain at least six feet (2 m) of 5/8 inch clear flexible tubing which has an outside diameter of 3/4 inch which is exactly the inner diameter of a standard garden hose. Cut into two lengths of 2–3 feet and slide the tubing inside each end of the hose 4–6 inches. It should form a good enough seal without any additional fuss. (If it doesn't, a wrap or two of Teflon, duct, or electrical tape should be enough.)
Lay out the hose assembly on the patio with each end held vertically with a stake into the ground (or chair, etc.) so it will hold water. (I find it convenient to use one inch brads through the tubing into the stake, but tape or string would work too.) Fill it full so the water level is about half way into the clear ends. Now you can easily determine the same level with extreme precision—perhaps 1/32 inch/0.8 mm at distances apart of 50+ ft depending on the length of the hose. Such accuracy is challenging (at best) with string and a carpenter's level.
To use it, place one end at a reference level and the other at a point in question. Raise or lower either end to cause the water height to increase or decrease, respectively, as doing so decreases or increases—respectively—the volume of the hose level. Once the reference end water level exactly matches the target height, mark the other end where the water is. Be sure to be consistent about using the top edge of the water or the bottom of the meniscus. It is entirely possible the hose has weeped a little water or it has relaxed or tightened due to temperature, sunlight, external moisture evaporation, etc. so recheck the reference end and re-level both ends if necessary and re-mark.
Arrange reference markers (such as more stakes) firmly into the ground at the corners and maybe some points along the sides of the patio and chose a reference height. It doesn't need to be at patio level. In fact, it is better to have it 12–24 inches above the patio level. Mark all the stakes at the reference level using the hose level.
From the reference marks all around, you can measure down to the actual height of the concrete pad or the cement forms. Do whatever slope calculation you desire, like 2.5 inches per 120 inches (a standard recommendation corresponding to 1/4 inch per foot) by measuring the distance from the house to the stake and applying the slope.
For example, if the marker stakes along one side are at 5', 10', and 15' from the house, the slope for each would be
5': (60 inches) is 2.5 / 120 * 60 = 1.25 inch drop
10': (120 inches) is 2.5 / 120 * 120 = 2.5 inch drop
15': (180 inches) is 2.5 / 120 * 180 = 3.75 inch drop
If the reference mark is 17.5 inches above the ground, then the final target marks should be at
5': 1.25" lower than 17.5" or 18.75 inches below the reference mark
10': 2.5" should be 2.5+17.5" or 20 inches below ref mark
15': 3.75" should be 2.5+17.5" or 21.25 inches below ref mark