Celtic have definitely clinched a top 6 spot. We still have to play Dundee twice.
Rangers have also clinched a top 6 spot. For Hibs and Aberdeen to overhaul them, Hibs would need to win out to get to 56, this would in turn mean Aberdeen would need to win out apart from against Hibs this would leave Dundee unable to over haul them due to losing to Aberdeen twice and Hibs. It would also mean St mirren needing to win all their matches apart from losing to Aberdeen which would see them beating Killie which would leave Kilmarnock unable to match Rangers current total due to losing to Aberdeen and St mirren.
For Aberdeen and Dundee to catch them it would require Aberdeen and Dundee to have one win each against each other but they both have to play Killie and St Mirren.
If Dundee and Hibs were to catch Rangers then Hibs would need to win all matches to get to 56, Dundee would need to win all matches apart from losing to Hibs to get to 56 and St Mirren would again need to beat Killie. Killies defeats to Dundee and St Mirren would leave them unable to catch Rangers.
So there you have it Rangers and Celtic have both clinched a top 6 finish.