If I modify the source ip address of all outgoing ip packets from my network to an ip address belonging to someone else (while ensuring that the checksum is correct) then what will happen.
Assume that I have a public IP address connected by a point-to-point link to an ISP.
Will the ISP check that the IP address in my IP packets is correct or will it just forward the packets.
I believe that ISP should just forward the packets.
what mechanisms are present in the Internet that prevent this from happening?
If you do this, you will probably be unable to establish a TCP connection — the replies will go to the spoofed IP address, preventing the three-way handshake from succeeding.
Your ISP may, or may not, employ egress filtering at their border routers and drop the spoofed packets.