Can we observe addresses, data and other signals on the microcomputer buses by an oscilloscope, a LED or even an ordinary DC voltmeter?
Build an "embedded debugger": wire a pushbutton, and write a piece of code that steps through your code every push of the button. The level of instructions per step should be settable, maybe via a potentiometer+thresholds. Or keep the uC connected to a computer and use an incircuit debugger supplied with your uC.
Reduce the clock and use LEDs (up to ~10Hz)/voltmeters (~1Hz). Cameras can film everything and the footage can be slowed down. Or use a parallel to serial converter and an oscilloscope (triggered on Chip Select, as Circuit fantasist said - or any single pulse that occurs in a read cycle) to see them all at once even at sort-of-high frequencies
Use a logic analyzer (Majenko was first to say it) - the advantage being that the data will be presented to you after decryption of the protocol and/or conversion of base (Serial, I2C, SPI, or in this case binary to decimal/hexadecimal).