There are several ways to detect portals in Nethack. Portals count as a trap and can be found any way that a trap could be. This is most important on the Elementals, where speed is key. The usual methods are:

• Read a cursed scroll of gold detection. This will show a pile of gold at every trap location. This is not so useful on the Plane of Fire, where there are many fire traps.
• Read a scroll of gold detection while confused.
• Use a crystal ball and search for ^. This may take an unacceptable amount of time.
• Zap a wand of secret door detection or the detect unseen spell, which will detect the portal if it is within a moderate radius.
• Wield or wear the Amulet of Yendor -- it will tell you whether the trail is hot or cold (only on the planes)
• Invoke the Amulet of Yendor, which will detect the portal if it is within a moderate radius.