You can check everything except the truck seal fairly easily by looking inside with a flashlight while someone carefully applies water from a hose to various parts on the outside. You have to look very carefully. Water can come in through a microscopic opening that you can't even see. In my experience on other brands of cars it is usually the tail light area. You might have to remove the carpeting etc to get a good view.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra