Built-in to almost all 3.7V 18650 Rechargeable Battery's Inhibits high current surges Protects against high-pressure, over temperature Resets and does not permanently disable the battery when triggered. However it's best not to trip them often as it irreversibly increases their electrical resistance by up to a factor of two and makes them more likely to have catastrophic failure. May not work when module included multi-cell series and/or parallel configurations.
From Capacity Normally the 3.7 Volt Lithium Battery's capacity won't exceed 3100 mAh for any reason(based the 0.5 C capacity-testing standard). If you got a 18650 battery, and labeled 5000 mAh or so, please don't risk your life to use it. It's always been this case. A grade 18650 cells for e-bikes and power tools. B grade for small assembly company of power bank product. C grade and those below C grade for flashlight uses. While Gpower manufactures their own A grade battery to their customers. From Price Some company will use LOW PRICE WITH HUGE CAPACITY to attract customers or even distributors. The price always be something around a 0.65 USD to 1.3 USD for one single 18650 lithium ion batteries. Now you dare to use it? No company will sell goods with a price which is much more lower than the cost. Besides now the labor cost is increasing in China. If there is, it could be bad ones.
From Size There's the risk to be deformed, leakage, explosion. Under the maniac price war, Some companies use low price PCB protection board or EVEN cut this part off. This kind of deeds make the market pay prices for the explosion-able 18650 batteries. It does not come easy to tell whether this a PCB inside. Here's a easy way but not very reliable way. The 18650 Li Ion Battery's dimension is 18mm by 65mm. 65mm is its length. When using in a flashlight, a PCB protected 18650 would be at least 67-68mm. Yet the most reliable way is to buy it from a reliable seller. From appearance For the original, it has a clear color and texture. Tidy and clean appearance. No scratch. It should be correctly labeled with Model No, Type, Capacity, Voltage, Negative and positive indications. Metal sheets no scratch, no blackened and greenish. If there's inconformity then you can tell it's not the original.