Always time the clutch engagement. I believe the limits are "between 60 and 100 sec", but more importantly, on any given machine it should take the same time on every start. If took 1:20min yesterday and 1:35min today, your belts are about to let go.
If it took 1:20min yesterday and 45sec today, your clutch actuator switches have failed and your belts aren't fully engaged. Both has happened to me.
Only exception is brand new belts(less than 40hrs or so), clutch engagement will take a few seconds more every day until they are stretched.
And, as general advice:
Human perception is a b!tch. We see only what we want to see or expect to see.
If you do the preflight with the attitude that everything is probably alright, you might go through the whole walk-around and "look" at everything, but wouldn't even notice if a tail rotor blade had fallen off. I've fallen into this trap a few times myself, missing things that should be completely obvious.
Instead, before doing the inspection, say to yourself: "there is something wrong with this machine and I'll find it!"