Not sure about NR, but in LTE in's all depends on CFI(Control Format Indicator), for this you have to read PCFICH channel.
CFI is a indicator telling how many OFDM symbols are used for carrying control channel (e.g, PDCCH and PHICH) at each subframe. If CFI is set to be 1 for a subframe, it means one symbol (the first symbol) at the subframe is used for PDCCH allocation. If CFI is 2, it means two symbols.
P.S - please go through PCFICH first, you will get good idea how PDCCH is being decoded by UE