The timing belt is a "service" item, replace every 60,000 miles, if it breaks, the engine stops working - ideally you replace it, before it breaks, not only because it can leave you stranded, but, on some vehicles, if it breaks, engine damage can occur - fortunately, the G13 engine in the Samurai is not one of these.
How do you know if it's broken - the engine will crank very rapidly and with a sort of flat whining sound, if you hear it once, you'll recognize it every time you hear it - it's unmistakable, but if you've never heard it before, remove the distributor cap and watch the rotor whilst someone cranks the engine - if the rotor is not turning when the engine is, there is a good chance the belt is gone - or - as Darrell suggests take off the belt cover.
Replacing a timing belt on one of these engines is a relatively simple process for anyone with basic mechanical skills and experience, you need to know if you fall into that category - I don't particularly care for "youtube mechanics", you should be able to find a factory manual over at Ack's FAQ (Ack's FAQ - Ack's FAQ
A timing belt kit containing the belt & a new tensioner pulley is less than $30 at RockAuto.com, I don't see why it should take you more than a couple of hours, I can probably do it in less, and I don't turn wrenches for a living, so I'd say $700 is ridiculously expensive, assuming that we are talking US dollars.
Some folks like to change the cam & crank seals, and the water pump, whilst they're in there, it's not a bad idea, the seals are a couple of dollars each and the pump is under $30, so if you're paying for labor, I'd say change them.