I had access to some testing software. We had software for "burn in" that supposedly tests the CPU and RAM, among other things.
My first computer in 1990 was a 386 DX 25 overclocked to 27MHz with a different crystal. It was, ahem, "factory overclocked" because the vendor sold them that way in order to "win" reviews against other vendors in Computer Shopper. It's still a trick used to this day with motherboard manufacturers that will surreptitiously set the CPU FSB one or two MHz higher so that their board will be at the top of any benchmark against a competitor.
486 based systems got pretty easy to overclock in the heyday with boards having tons of jumpers onboard. I remember having OPTi and ULi chipset VLB boards (VESA Local Bus) with jumpers for 25/33/40/50MHz FSB, 1/2/3/4x multipliers and various voltages. The first CPU I killed was a 486SX25 that I tried overclocking. Was running it at 40MHz when it suddenly locked up. It was dead from that point on. No great loss as Pentiums were already out, so this was just an old CPU we had gotten for free and were screwing with (me and NowhereMan, for those that remember permanently banned members).
Got better with socket 7. OMG, the TX chipset Shuttle HOT-557 was probably the best, with ettings available in BIOS and a jumper block (replaced by dip switches in later boards) for all the BUS speeds and multipliers overriding the BIOS. I think this was the first Anandtech review I read, BITD. These rocked with any Intel CPU. Pentium 75 was an easy 90MHz. Pentium 90 was an easy 100MHz. Pentium 100 was an easy 120MHz. 120=133, 133=150, 150=166. Sometimes if you lucked out you can jump two speed grades stably. The best were the Pentium MMX chips. They were available for the desktop in 166/200/233MHz models, and a 266MHz notebook model. They were such easy overclocks that many unscrupulous vendors sold Pentium MMX 166 chips in systems overclocked to 233MHz, and there was even a cottage industry around "remarking" the chips meaning sanding the print off it and printing the markings from a faster CPU on it. Since the bus and multiplier were conpletely controlled by the motherboard, it was impossible for a normal user to tell that they were paying extra for a slower CPU. I once even got a Pentium MMX 233MHz running at a whole 300MHz on a super 7 board, but it suffered an early, mysterious and sudden death for some reason. :shocked: Ah well, belonged to a friend of mine and he got it for really cheap since we were already at faster slot systems.
I moved on eventually to an Asus P5A board that was an ALI chipset or something, with a bunch of FSB speeds. It was my first ATX board and a rockin' "Super 7" board with an overclocked AMD K6-2. I even had a Matrox M3D 3D accelerator and a Matrox G200. This was probably my last machine that I regularly booted into DOS, because Quake 3 came out and my life changed.