How do you know that the piston top is facing right direction after put in block?

To ensure the piston top is facing the correct direction after being placed in the block, check for any alignment marks or notches on the piston and block that indicate the proper orientation. Additionally, the piston usually has a specific shape or design (such as a valve relief) that should align with the cylinder head and valves. Finally, confirming that the piston rings are correctly seated and not twisted can also help verify proper installation.