Nothing - NOTHING - is foolproof. People get killed every year by "foolproof" things. There are lots of ways to improve your chances of NOT becoming one of those statistics, though.
First and foremost... any time you or anyone else will be underneath a vehicle, make sure that something else is under the car with you - something THICKER than your head, THICKER than your chest, and strong enough to withstand the full impact of your vehicle falling on it WITHOUT yielding. It needs to be wide enough to spread the falling vehicle's load over plenty of ground surface - if it's thin, the falling vehicle may simply drive it into the ground like a stake. I like big truck wheels under a vehicle with me - they're large, they're thick, they're astoundingly strong.
DO NOT use concrete blocks as sacrificial "vehicle catchers", and ESPECIALLY don't use them with the webs horizontal. They simply won't tolerate either the impact or the weight. Not reliably.
Even big commercial pneumatic or hydraulic or pneumatic-hydraulic lifts should not be used to hold a vehicle in the air while people are under it UNLESS a failsafe is used to catch the vehicle in the event the lift's valves fail. The same applies to chain-operated lifts; chains can and do break. Again, I've seen commercial lifts drop vehicles.
Ramps... are useless and dangerous IF they're not built heavily enough to easily handle the vehicle's resting weight. They're also useless if the ground is so soft that they can be pushed into it by the vehicle's resting weight, or if the ground is so sloped that the vehicle can tip (in any direction) off the ramp, or tip over the ramp itself. Don't laugh, I've seen ramps fall over.
If using ramps, always chock the wheels that are NOT on a ramp. It's a good idea, too, to set chocks ahead of those same wheels just before you drive up a pair of ramps, with the chocks carefully laid out at the right distance such that the non-ramped wheels come into contact with the chocks just as the ramped wheels are ready to roll off the far side of the ramps. Again, I've seen people overdrive their ramps, resulting in the car looking silly with ramps under the doors instead of under the wheels.
Likewise, jack stands are useless and dangerous if they're not built heavily enough for the vehicle, or if they can be pushed into the ground, or if they can fall over.
One of the safest ways I know to lift a vehicle a short distance is to use short pieces of wood planks stacked up to the necessary height. The planks need to be on a level concrete floor, and they should be AT LEAST as wide as the tires they're supporting. If long enough, they can be used as simple ramps, even. Even up on stacked planks, though... you should never be under there without a "catcher".
Don't screw around with your life. If you ever catch yourself thinking "that's probably good enough", rethink. If you're ever going to get old, it should be several times as good as it NEEDS to be... and there should always be AT LEAST TWO solid things between that car and your headstone.