You're wanting to know how to check out the hull, right? If so, it sometimes helps to know how the boat was constructed. If it's a fiberglass boat with a wood core, you'll want to check for rot, especially in the transom. The transom (and the deck) are the most common places that wood will rot. You'll want to feel around for soft spots, as soft spots usually mean a rotten core. You can also bang around with a rubber- or wood- mallet. If the sound is a sharp rap, it's good. If it sounds "hollow", you may have a soft/rotting core. Also, look at the nut & washer that hold the outboard or outdrive to the transom. If they look like they are indenting the transom, they are almost surely smashing a rotted transom. Also, if they are "weeping" and leaving a dark drip line - that is a sure sign the transom is wet and will be rotten if it isn't already. Pay special attention to the stringers. If they are rotten, the boat will not be sufficiently rigid, especially in rough waters, when you really need it to be rigid!
Also, look for substantial cracking. Spiderweb cracking of the gelcoat isn't a problem, and is almost always present in an older boat, but if they are deep (and especially if they are wide) cracks - they may be structural. If in doubt, walk away.
Obviously, keep an eye out for anything that's crumpled-looking. If it's been wrecked, don't get it.
If you're going to drop a substantial amount of cash on the purchase, you may well want to consider a professional survey. Unless you're looking at a major league vessel, you can get away for $500 or less on a survey. The survey will also help when/if you decide to insure the boat, so it's a "two birds with one stone" proposition.