Halvings are determined by blocks not by any precisely specified time and happen every 210,000 blocks. So the next halving will be at block 740,000.
A little correction from me, the correct one is at block 840,000
The easiest and most valid way is, open the Bitcoin blockchain explorer website then wait until the 840,000th block arrives, that's where the halving moment occurs.
For example, I opened mempool.space, it will look like the image below

Currently, the Bitcoin blockchain is in progress at block
833111, which means it still needs
6899 more blocks to reach block 840,000.
The estimated process for one block is 10 minutes, but that is sometimes faster and sometimes longer, so the estimate will always change depending on how fast or slow the block completion process is by the miner.