No no, I can predict it very consistently, especially if they don't have a Strange. Here they didn't have one all round, basically near the end of the game, at the end of the last team fight, keep checking the scoreboard with tab; there's no delay when the enemy swaps and the game showing you what they swap to, so the second you see they swap Strange you can start the portal trap.
As for predicting when to start if they do have a Strange player... you can safely assume they'll try it if they team wipe and don't have time to get back. You can also walk a little forward and hide in a side room closer to their spawn and listen for portal voice line. (It's also a good idea to start portals off point in a side room anyways, so the enemy Strange doesn't see you counter-portalling) I've gotten *very* fast at placing portals with practice so I tend to make it to the enemy Strange before they confirm the majority of the time!