The Multi-Meter is probably one of the most valuable items in your tool bag. Knowing how to use it properly is very important. Metering for DC Voltage and Resistance is the most common application, but don't cheap out on your meter. Treat that thing like it is your child!
Having a Multi-Tool Screwdriver is extremely important because you absolutely never know what you are going to find out in the field. For most applications, a standard 6-in-1 screwdriver should do the trick, but in more rare instances, extra tools may be necessary, such as oddly sized hex heads, and sometimes Torx style screw drivers too.
Tamper-Proof Bit Set
A tamper-proof screwdriver set is very important for sprinkler devices. Regular testing should include functional testing of the water-flow switch and tamper switches, but should any of these ever need to be serviced in any way, you will need to have the special tool to open it up.
Crimp Connectors / Crimps
All kinds of crimp connectors (and wire nuts for that matter) as well as crimping tools will ensure you have everything you need in the event any crimped wires should need to be replace. The most common place you will find wire crimps are on battery leads of the main fire panel, booster panel, or basically anywhere you can find a fire-alarm battery. These often need to be replaced when battery corrosion occurs, or if the crimp has been worn down to where it is too loose.
Remember: You need to work with all kinds of low voltage wire, and sometimes even the high voltage. Be sure that when placing in a wire nut or a crimp, no metal is ever exposed. The insulation is there for a reason, and exposed wiring could cause a short circuit or ground fault. Neither of those are fun to troubleshoot.
Test Smoke & Test CO
Test Smoke. Lots of it. When doing any inspections, most will often go through several of these cans a week. Per NFPA 72, functionally testing a detector is mandatory. Even if a smoke detector can activate with a magnet or test button, those features only test to see if it is reporting to the panel, but does not ensure proper functionality. Also note that CO test cans are available as well.
Jumper Wires are always going to be helpful to have around. When troubleshooting, they will help in shorting out a zone, temporarily bypassing a relay that cannot be disabled, and so many other uses you will discover along the way. Having them around will save a lot of time and headache.
Start collecting keys! Fire alarm keys can be purchased from online stores such as eBay, and often spares can be found on job sites. I would not suggest relying on any site to provide keys for you. Every brand has a different key, and sometimes multiple keys are necessary in one building. A heavy key ring is a reliable one!
Don't forget hex wrenches! Some pull stations, such as the BG-12, use a hex wrench to reset rather than a real key. Luckily, we are moving away from that for the most part.
These are just the basics, but you will find so many important things to have on hand all the time, such as a tablet or laptop. Pen and paper may be a good thing to have on hand. Think of all the little things, because being prepared for anything will make your life much easier out in the field.