I believe GP used assembler to refer to the program which reads your hand-written assembly and produces a binary. That program was presumably given to you so you need to trust it.

No, the only way to solve this problem is to start with a computer entirely devoid of code and bit-bang your assembler into the machine with switches, the way the first users of the MITS Altair did it.

It might be, but have you also validated the microcode executing it?

Or the Verilog/VHDL for the logic gates used by the CPU, for that matter?