Given the age of the car and that diesel fuel turns to sludge, I'd pull the tank and have it cleaned. I'm not concerned that diesel will be mixed with gas, more with the gas breaking free sludge then clogging filters.

If you don't get it cleaned, at the outlet of the tank install a clear fuel filter and watch it for sediment. Be sure to have fuel flowing in the proper outside - in direction so you can see the dirt. I really don't like clear plastic fuel filters in hot areas like the engine compartment so a take apart glass one would be better if you want a see through one at the engine as well.