You can check for T-belt slippage fairly easily. Turn the crankshaft until the timing mark on the damper pulley lines up on TDC. Then remove the upper T-belt cover and see if the cam sprocket timing mark is dead on. Sometimes it's kinda tight getting the plastic cover pulled up from behind the water pump pulley. The cover is held on with a couple bolts from the front AND a small screw coming in from the back side at the very top of the cover. This photo shows where the timing marks are:

http://www.brickboard.com/FAQ/700-900/B230FTimingBeltAlignment.htm

Note: You won't see the crank shaft timing mark as in the photos because your damper will still be mounted. So you'll have to rely on the damper mark. Here's a catch - the damper pulley is made of an inner "hub" and an outer pulley ring with a rubber isolator ring between the two. Often, with old age, the outer ring has spun on the rubber ring which renders the damper timing mark useless. You can remove spark plug #1 and with a long screwdriver determine if the piston is at TDC, then see if the damper mark is at TDC, If it is, then the pulley hasn't spun and you can rely on the damper mark.

If you have a cam-driven distributor mounted to the rear of the head, you can ignore the timing marks on the intermediate shaft that runs down the LH side of the engine - all it does in this case is drive the oil pump. If you have the distributor mounted on the LH side of the engine, then the I-shaft timing IS critical as it drives the distributor also. In that case you have to remove the damper pulley and lower T-belt cover to see the I-shaft marks. (The job just got bigger.)

Check to make sure the belt is fairly snug. It's kinda sounding like they didn't replace the tensioner and maybe should have, or they don't have it mounted nice and flush up against the engine block - a common boo-boo.