If your engine starts running rough. My Tacoma had the same problem. I drove it around for a year with a leak and then the truck started running rough. By that time it was too late. It cost me $1500 to have my 4 cyl head rebuilt (3 burned out exhaust valves) and have the exhaust manifold replaced. The good news is that the truck had 165k miles on it and I just sold the truck with 206k on it. The head is the only work I had done on the truck.
You could do a compression check on the truck, but I would just take the truck into a good mechanic and ask to have the leak found and your heads checked for valve burnout. A junkyard exhaust manifold should be pretty cheap, maybe a $300 fix?