How can I know which process is using swap?

Improving the cyberciti.biz command to show a more concise answer:

(echo "COMM PID SWAP"; for file in /proc/*/status ; do awk '/^Pid|VmSwap|Name/{printf $2 " " $3}END{ print ""}' $file; done | grep kB | grep -wv "0 kB" | sort -k 3 -n -r) | column -t

An example output:

COMM             PID    SWAP  
dockerd          662    2736  kB
skypeforlinux    26865  1320  kB
NetworkManager   303    1112  kB
slim             392    1028  kB
redis-server     350    204   kB

From here:

[a] /proc/meminfo - This file reports statistics about memory usage on the system. It is used by free to report the amount of free and used memory (both physical and swap) on the system as well as the shared memory and buffers used by the kernel. You can also use free, vmstat and other tools to find out the same information.

[b] /proc/${PID}/smaps, /proc/${PID}/status, and /proc/${PID}/stat : Use these files to find information about memory, pages and swap used by each process using its PID.

[c] smem - This command (python script) reports memory usage with shared memory divided proportionally.

Also you can refer Find out what is using your swap

#!/bin/bash
# Get current swap usage for all running processes
# Erik Ljungstrom 27/05/2011
SUM=0
OVERALL=0
for DIR in `find /proc/ -maxdepth 1 -type d | egrep "^/proc/[0-9]"` ; do
PID=`echo $DIR | cut -d / -f 3`
PROGNAME=`ps -p $PID -o comm --no-headers`
for SWAP in `grep Swap $DIR/smaps 2>/dev/null| awk '{ print $2 }'`
do
let SUM=$SUM+$SWAP
done
echo "PID=$PID - Swap used: $SUM - ($PROGNAME )"
let OVERALL=$OVERALL+$SUM
SUM=0

done
echo "Overall swap used: $OVERALL"