Joists of uncovered structures tend to rot at the top, where water sits. Probe between floor boards to check for rot more than roughly 1 .8 inch of penetration indicates weakened lumber. Check also at joist ends, and the undersides, especially if the space below has restricted ventilation. Look for mold and fungus on the surface it may be black or white.
The outside (header) joist of a porch, deck, or balcony is more susceptible to rot as a result of exposure to moisture. The ends of joists which rest in pocketed masonry can rot quickly.