public boolean isEpochBlock(final long blockNumber) {
    if (blockNumber < 0) {
      throw new IllegalArgumentException("Block number must be 0 or greater.");
    }
    if (blockNumber < startBlock - 1) {
      return false;
    }
    return (blockNumber - (startBlock == 0 ? 0 : startBlock - 1)) % epochLengthInBlocks == 0;
  }
Is epoch block.

@param blockNumber the block number
@return the boolean
consensus/common/src/main/java/org/hyperledger/besu/consensus/common/EpochManager.java