```
bitcoind -par=1
```

Excerpt from the `debug.log`:

```
Using 0 threads for script verification
```

Zero? I guess not ;)
#if defined(HAVE_CONFIG_H)
#include <config/bitcoin-config.h>
#endif
#include <init.h>
#include <addrman.h>
#include <amount.h>
#include <banman.h>
#include <blockfilter.h>
#include <chain.h>
#include <chainparams.h>
#include <compat/sanity.h>
#include <consensus/validation.h>
#include <fs.h>
#include <httprpc.h>
#include <httpserver.h>
#include <index/blockfilterindex.h>
#include <index/txindex.h>
#include <interfaces/chain.h>
#include <key.h>
#include <miner.h>
#include <net.h>
#include <net_permissions.h>
#include <net_processing.h>