John is right. But you can usually see if it’s even worth wasting time setting up a test circuit by using your DMM or LCR tester to measure resistance between Vcc and either Output pin. Fake chips will usually measure open or significantly off from the DCR reading you’d expect using that measurement. For reference, a real NE556 measures around 12k DCR between the two (pin 14 to either pin 5 or pin 8). Anything very far off from that and you’re probably best moving on.
Doesn't always work, depending on which original IC was sanded down and relabeled, but it does most of the time.