The quick answer is to find your lowest temperature over a number of days at root depth. This usually occurs just before the sun would normally hit the location you have in mind. Go at least two inches down to make sure you are measuring the temperature the soil will stay at if there is no sun for a couple of days.
One way to do this is to bury a mason jar with water in the soil in a representative location, and float an aquarium thermometer in the water and leave it there. Check early morning and record for a number of days to get a moving average. A digital probe thermometer will work as well and will come with instructions.
If you wait until conditions are ideal to plant squash then half the season will be gone before you have the courage, so you might want to consider planting in hills. By pulling the soil up into a mound 3 to 4 inches high and planting 3 or 4 plants in it you allow the plants to get cool but their roots will be free of cold water which is the goal. This is particularly important for heavy soils. You can achieve the height by digging out a pit for each hill and incorporating well rotted manure and other rich compost as you refill the hole. This will raise the soil level, improve drainage, and darken the soil to absorb more solar radiation.
