OK, back to the question.
I have some stems on bikes and in the spare parts box (all alloy) that have torque specs printed on them, but honestly, most do not. The marked ones vary and are 5 to 8 nm. For several years, I've only used the Ritchey Torque Key (5 nm) on every stem I own (both steer tube and handlebar clamps)** and it works - no slippage. If it did slip, I'd just use that tool to get it to that torque, then just tighten a little at a time with a regular hex wrench until it was tight enough. That's one way to go and you'll be safe.
Before that, I just tightened conservatively using a small J-shaped Allen key. I mention the size of the allen wrench because the smaller the size, the less apt you'll be to over tighten a ridiculous amount. I'd then "test" by putting a lot of downward force on the the hoods. Sort of like what would happen if you hit a good bump while riding on the hoods. That, really, is the only danger of slipping handlebars - you go down because your bars slip.
So the simple way to do this is: tighten conservatively (use an X pattern). Give it a good push downward on the hoods. If they slip, tighten a tad more, re-test. If the bars and/or stem are carbon fiber, use assembly paste.
Like all things using small fasteners, just don't be a gorilla and use common sense.
** for what it's worth, the 5nm Ritchey Torque Key comes with a 4mm hex bit in it. It's pretty easy to swap it out for a 5mm if you need it - just heat it up (heat gun, hair dryer, candle? match?), grab the hex bit with a vice grips or such, and pull it out.