For a non-Calif 1990, you should have the 16-pin connector over by the battery. Most of the pins are blank, but pin 1 is ground, pin 3 is the CIS, 6 is SRS, and 7 is AC.
Do a search here for "error code reader and reset tool" or similar subjects.
There is one I found by a fellow named WES that outlines the little box with a switch and LED that you can easily build. Plug it into the connector and record flashes. Flashes vs codes are also referenced in the thread. I built one awhile back, and it works great.

One comment - the CEL will not light up in most cases, even if you have a dead sensor or other major problem. The only time it will light is if the problem is severe enough that the system cannot still correct the fuel mix to a normal range. However, by reading the codes the problem will show up.
I've disconnected various temp and throttle switches at times, no CEL - as long as the system could default around them and get the fuel trim back to semi-normal.

DG