How big is the harness, where is it located, what does it look like?
How do I know if it is bad? or good?

Is there a way to know if my distributor/rotor is bad or do I just replace it and see?

 

Very big.

All over the car. Under the hood, under the dash, in the doors, in the trunk, under the seats....

Like a bunch of wires all bundled together and wrapped with plastic tubing or electrical tape to keep everything tidy, with terminals, plugs, sockets, and assorted end-of-wire things located just where they need to be.

Older Volvos had a problem with the plastic insulation on the outside of the wires falling apart, allowing the copper conductors inside to touch other wires, "shorting" out various circuits. This could blow fuses (inconvenient) or even start fires (much more inconvenient). The problem happens the most along the firewall (rear area of the engine compartment) where the wires are exposed to engine heat for years on end.

Here's a picture (from a Mercedes site) that shows a Volvo 240 plug, and what happens to the wires...



That's a whole other topic, but if the car isn't running quite right, it can't hurt to pop off the distributor cap and have a look. If the terminals in the cap or the tip of the rotor look pitted and worn, it's probably time to change them, and maybe do the wires and spark plugs too, if you can't remember when they were last changed.

You could also go to Google, pick the "Images" option at the top, and type things like "worn distributor cap" or "worn distributor rotor" to get an idea of what the bad parts look like.

 

Look on your firewall under the hood. You should see a bunch of wires and on either end a grey plug...