Your best bet is to log into your modem and look at the ARP table to see what devices have recently talked on your network, and look at your DHCP pool to see what devices are currently assigned addresses.
can anyone hack your computer through WiFi?
Yes-ish. If you are running out of date installations or have weak user passwords on your PC then a remote compromise can happen.
If you think someone has compromised your wifi, first thing is to change your wifi password and router password. 12 characters including uppercase lowercase letters, numbers and symbols.