Besides the other tips you can also use that little box in the N2 troubleshooting guide (can be found on JCI quicklit or just google for it)... It'll work the same on BACnet. It's only. Handful of parts and takes maybe an hour to construct...
What symptoms are you getting? One thing that can drag the bus down is looping the wire in circles say like at a VAV Box with the mindset that maybe someday the box may get moved a few feet.
Other things to look out for are would be using the correct wire size for the baud rate. Some old timers are so used to using 18 gauge wire that they think it will work just fine at 38,400 or 76,800. Not so.
I prefer to do things by the book and then if there is a problem start like TT suggests and split the bus until the problem goes away. I learned to trouble shoot wiring problems when I first started out in controls and fire alarm systems using that method.
Do a Google search for MS/TP Communications Bus Technical Bulletin. It is a JCI Document and it has some good basic info in it. If you can't find it PM me and I'll send it to you.
If you can't fix it with JB Weld, Duct Tape, and Ty Wire it has to be replaced.
No good deed goes unpunished.
If you want to take off friday to go fishing then make sure you train your helper right.