Until the latter part of the 20th Century, the most common observational method to find one's latitude was to use a sextant or octant and take a noon shot of the sun. The noon shot was preferred because 1) an accurate timepiece was not necessary and 2) an observation taken when the sun is due north or south (that is, on the observer's meridian) greatly simplifies the calculations. Let's use Lewis's observation at the Three Forks to show how the captains made their observations and calculations to determine latitude.
