Many CPU's can be manipulated to run at higher clock speeds (overclocking). That's why sometimes retailer decide to relabel CPU's for higher clock speeds and sell these for the price of higher clocked processors. This occurs often with top of the line CPU's because these offer a large profit gain.
Another cause for remarking is down binning by the manufacturer. That leaves definitively room for higher clock speeds even within original specifications.

Following a list of known remarks:

intel Pentium 90MHz and 133MHz
Both versions come in ceramic package (CPGA). The CPU top gets lapped and is reprinted with different specs. The procedure seems OK for both processors as these were downbinned by intel to fill marketing gaps. The lapping can be detected by inspecting the lower left of the CPU (which is marked with a grey point). This should be lower than the rest of the ceramic package. After lapping the top it's the same heigth as the rest of the package.

intel Pentium II 233MHz and 266MHz
Both versions can be overclocked to 300MHz without major problems. But most of these CPU's run unstable over time because heat dissipation is kinda high. Even L2 cache RAM's used inside the CPU cartridge sometimes were not specified for clock speeds of 300MHz. A software tool exists to detect remarked CPU's. It works by detecting timing of L2 cache and ECC support (which was used with 300MHz versions).
Furthermore, according to intel the serial numbers 98070742-700 to 98070742-702 that were found on Pentium II fakes do not exist. But Intel gives out information about serial numbers only to retailers, not to end-customers.
Pentium II 233MHz Boxed version
Get a pair of perfectly genuine PII-233s, one boxed version (SL2QA [CPU ID 6-3-4] w/o ECC), one bulk version (SL2HF [6-3-4] w/ ECC).
Remove SL2QA from box. Keep all original bundled items (booklets, warranty, logo, fan cable etc.) remaining sealed.
Short out on-chip anti overclocking regulator on SL2HF.
Attach heat sink/fan assembly from one in SL2QA to the one in SL2HF.
Attach a counterfeited outer casing with absolute clean, sharp, and legit markings for a PII-300 in SL2QC series.
Put a new '300' with all genuine & sealed bundles in a brand new but counterfeited color box with all the legit sayings for PII-300 in SL2QC
Shrink wrap the sucker, and here we have a perfect 'PII-300' in full color & sealed retail box
As for the remaining SL2QA (the one w/o ECC), two ways of reselling exist:
I. Resell as a PII-300 bulk (SL2HA/0634) regardless of any detection.
II. Resell as PII-233 bulk. It's the safe way.
Pentium II 233MHz bulk version
Get a perfectly genuine 233 in SL2HF (bulk 0634) series equipped with ECC L2.
Remove outer plastic casing of SEC cartridge
Short out on-chip anti overclocking regulator
Replace the outer plastic SEC casing by counterfeited one with absolute clean, sharp, and legit markings for a PII-300 in SL2HA (bulk 0634) series.

intel Pentium II 266, 333 and 450MHz
PII 333 -> PII 350 (two resistors changed R5 1 kOhm, R6 3,3 kOhm)
PII 300 -> PII 450
PII 266 -> PII 400 (two resistors removed R5, R6)
Click for large view

Pentium II SL2S8 is definitely a remark because intel never used that S-spec for Pentium II.

AMD K6 233MHz
There are 3.2V and 3.3V versions of the 233MHz K6 processor. The 3.3V chip was supposed to only be distributed to OEMs. So if you buy a boxed K6, it shouldn't be the 3.3V version. Also, some boxed 3.2V versions were incorrectly marked as 3.4V. These chips are genuine, and should be run at 3.2V.

AMD K6 166MHz
AMD doesn't seem to care much about the fact that remarking their chips can be done very easily. There's only some paint printed on the aluminium heat sink, which can be wiped off with several chemicals. Printing a new marking on the heat sink is not too hard to do. This makes a K6 233 out of a K6 200 or even K6 166. Be careful with that, examine the printing very carefully. If it's done well, you won't have a chance though.

AMD K7 Athlon 650MHz
The chips were marked as Athlon 650 0.25µm (C types) with serial numbers on the case of 219936022369, 219936022600 and 219936022907. Inside, the serial number on the circuit board had been removed and replaced with a smaller sticker with different printing. The serial numbers on the inside sticker differed slightly from those on the cartridge: 219936022781, 219936022780, 219936022802.
In addition, the metal clips holding the CPU circuit card looked like they had been opened and reclosed, while some of the resistor pads (R121, R147, R158) on the circuit card showed signs of being removed or resoldered.