take out the starter relay, if you measure one lead to ground the other to the socket pins(where the relay was) two should have battery voltage, from the PCM fuse and STARTER fuse. one of the other ones goes to the starter small wire. you can measure continuity. the other is the ground control from the pcm, so when you first test it it may seem like there is 12v on it but if you have someone turn the key to start it should go low, or close to 0 volts. there is another fuse to check as well the "crank" fuse i think on the 2000+'* its under the back seat. if its good you should test it for 12v to ground when the key is in start.
you test wires by "backprobing" them. which is taking a sharp probe that pierces the insulation and can see what the wire has on it, you always want to save that for last resort though as it leaves holes in the insulation and is asking for corrosion to get in.
let me know what you find i can help you further.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock