Rifter #14 includes the ruling (reprinted online here) that
For attempting a task that they are unskilled in, a character should roll percentile vs. an applicable attribute (i.e. IQ, ME, PS, PE, etc.).
In other words, most skills will roll d100 and success will be if the number rolled is below the PC's IQ stat. For most people, that means unskilled actions will succeed about 10–20% of the time.
Something involving mental fortitude (is there a Torture skill somewhere?) would fall under ME. Something involving charming would fall under MA. Swimming might fall under PS or PE. Something lithe or graceful like dancing would use PP. (GM ruling as to whether that or PS better captures climbing; my own take would be that some forms of vertical climbing would use a PP roll but require a PS at least able to carry the PC's own weight; sheer faces would just be impossible without tools, magic, or natural abilities of some kind.)
Officially, perception checks are handled separately, so ambushing or prowling against an active observer gets treated as a modified combat roll.
It's important to remember, though, that standard actions don't require any roll at all. Per Rifter No. 14:
In most ordinary circumstances, just possessing a skill will allow you to perform basic tasks without having to roll for success. I.e. with the Pilot Automobile skill you can drive a car under normal conditions without having to roll under the skill. Of course when under stress or performing under difficult circumstances, a skill roll wou[l]d be needed.
The G.M. should decide when and where a skill roll will be needed for basic tasks.
Generally "taking 10" in most actions is an explicit assumption of Palladium's low beginning levels. New pilots don't crash their armor 60% of the time they fly; they only have to run checks when they try anything too fancy or (e.g.) evasive maneuvers under extreme duress. People don't forget how to speak their native language every Thursday; they just can't remember an obscure word, mimic a particular dialect, or explain a point of grammar 100% of the time.
A PC trying to drive a car without a skill should be able to get home, as long as s/he's going 5–10 mph, not intoxicated, not being attacked, and not driving over very rough terrain. If things become difficult, it'll be time to roll against IQ or PP and start looking up the SDC value of trees. Similarly, if skilled players need something difficult or obscure that their skill level shouldn't cover (e.g. trying to evade Chitown security in a getaway or using First Aid to deal with an alien virus infection), the GM can similarly just rule that it's time to go out and find an actual expert or mage who can handle it at the necessary level.
Similarly, you don't get levels by backflipping repeatedly for +25 xp a pop. You get that xp when you successfully use skills in difficult or game-appropriate situations. This is also covered in Rifter #14, although not available online:
The 25 experience point award for using a skill should generally only apply to meaningful use of a skill, not just any use. This means that driving to work or around the town does not count as using a skill for the experience, but being involved in a high speed pursuit which may require multiple control rolls and driving skill checks counts for experience from skills.