Important Fire Alarm Tech Tools for the Truck
Smoke Test Poles

Test poles of all kinds are important. The most common items include the Solo 330, which uses smoke cans, and the Solo 365, which uses smoke cartridges. Either one of these, or comparable product by another brand, are great options for testing smoke detectors of just about any brand. This unit paired with a strong extension pole makes testing high up detectors very easy!
Heat Test Poles

The heat detector tester, similar to the smoke detector tester, will also pair up nicely with an extension pole, allowing you to functionally test heat detectors that allow for rate-of-rise, resettable operation. It's important to note that some conventional heat detectors will pop if they get too hot, and cannot be reset. For more information on that, check out this page.
Ladders

Then of course, ladders. For different job sites, different ladders will be necessary. Sometimes a 3 foot, sometimes an 8 foot, but then of course we have our other applications. Pictured is the "Little Giant" ladder, which is fairly heavy, but can be small when needed to be small, or extend very tall where it's necessary. Regardless of the kind of ladder you have on hand, always be sure it's the right one for the job, and use it safely.
Drill

A standard drill, and even maybe a dremel and drywall saw, is a good thing to have around should any service call require one. Cordless drills are recommended, so be sure to keep any batteries charged up at all times. If you aren't an installer, chances are you won't need to be making holes in the wall too often, but that's not to say it'll never happen, because it likely will.
Vacuum

Speaking of making messes with drills, it's a good idea to keep a small shop vac in the truck so that in the event any of your work leaves a mess around your customer's site, you can clean that up for them nice and quick.
Extension Cord(s)

With power tools comes the need for power. Bring an extension cord with you. There is not always going to be an outlet in a close, convenient location. You will find that an extension cord will also help you keep your phone, tablet, or laptop charged up through out the work day as you enter in your paperwork.
Spare Parts

Finally, always keep your truck full of common spare parts. You never know when you may need it. Detector not working? I'll replace that right now. Horn/Strobe broken? Good thing I have one that'll sync with the rest!
Common parts to carry would be, pull stations, smoke detectors, heat detectors, modules, horn/strobes, FPL wire of different gauges, screws, conduit and fittings, and of course - batteries. Remember to carry both conventional, and addressable devices that are compatible with the systems you may encounter.
Other
As always, you will find that these are just the very basics, and there may be more you have on hand at all times. Regardless, keep your truck clean and organized so you can find whatever you need quickly. Try not to keep the customer waiting!