Having had twins, I can’t imagine it happening 6 weeks in for more than a few minutes. 6 hours, definitely. 6 days, maybe. But by 6 weeks, you know who is who. Even identical twins are pretty easy to tell apart after having spent significant time with them. It’s actually pretty common for parents of di/di identical twins (which are the type that could be fraternal) to not think their twins are identical only to have everyone else notice they are.