Not the SLOPE as much as a sudden CHANGE.
Look for roots, rocks, etc on the high side, look for dips and gopher holes on the low side.
Do NOT be staring at a goofy gauge.
If it feels bad then it IS bad.
Seat belt if you have ROPS, no seat belt if you don't have ROPS (better chance of being thrown clear).
Keep the bucket and anything on the 3pt low and be ready to dump them QUICKLY.
Probably the HARDEST thing is to steer DOWN the slope if/when things get really hairy.
However counter intuitive this may seem at the time, steering UP the slope is more likely to roll you.
REG has it right. Especially watch for what might become a sudden change such as a hole on the lower side, a groundhog hole complex, a small log that will roll with you, a big rock, etc.
1) Keep the bucket and anything suspended from the tractor low as possible.
2) If you can't go up it in 4wd due to wheel spin then you can't control your machine on that slope. Test it.
3) Steering to the downhill is your main exit strategy and you better plan for when that is or is not feasible. Not feasible if it leads to something worse like water or a steeper slope or trees.
4) I'm not for seat belts. Cars YES, tractors, NO. Watching the Penn State safety demo at Ag Progress days in PA the dummy on the remote controlled tractor is OK after the tractor rolls over because he had the belt on and his ROPS up. (Great demo!) The problem is there is an infinite variety of situations and being strapped down by anything amounts to a hazzard. It is a tough call but being ready to jump appeals to me. Still, I have lost a few friends to tractor rollover -- none wearing seat belts and all VERY experienced. Not a simple choice. You have to make your own call.
5) I am told that 15% slopes are some sort of ISO/ASME spec. If that is true then throw away ISO specs. Hill country farmers are routinely found on slopes exceeding 40% without even giving it much thought. I measured a couple of places considered "steep but no where near prohibitive" and definitely very safe for a decent tractor and they averaged 40%. I've operated 4wd tractors of JD and MF brands on considerably steeper slopes than that (but only up and down) with no problem. The 40% slopes are OK sideways if your rear wheels are out enough. Tire spacing and cg height are the whole ball game for going around hills sideways. CG is hard to measure or know.
6) You better have brakes that will hold the machine. I often find it compelling to ease down some VERY steep spots using brakes in 4wd where all 4 tires grab. You can let yourself down over sudden drops, shelfs, etc. safely where "clutch out motion" is not safe.
7) While keeping that bucket low, be careful not to have it so low as to dig in going down a steep slope ! A sudden unplanned digging dive of the bucket can ruin your day as well as end your life. Keep the bucket low but "Tips up" is good advice in both tractor operation and skiing.
8) Can you keep your engine running ? There are tractors on the market, brand new, that (until the mfr comes out with product improvement kits) will starve for fuel trying to go up a 40% slope while carrying 1/3 tank or more of fuel. Dying engine and lousy brakes are a bad combo on the steep. Was that a pond behind you !?