Depending upon failure mode, you may get exhaust gasses in the coolant. Sometimes you can see tiny bubbles. Shops can use their exhaust sniffer to diagnose.
Other potential symptoms have been covered well.
Sometimes gaskets holds, but head cracks. Or gasket fails AND head cracks as well. Symptoms are pretty much the same. If you do need a head job, be certain to have the head checked for cracks. Learned this lesson the 'hard' way in my youth...
Of course already you know the short answer is; "don't do that".
Trying to limp those few extra miles is a high stakes gamble.
Tow and rescue cost is cheap insurance.
1986 300E 5-Speed 240k mi.