747 - Look for the hump. It's also the only four-holer in DL's fleet.
A330 - Look for the little raked wingtips/winglets that have a sharper angle than the rest of the wing, but aren't raised very high above it.
777 - Looks like an A330, but without the raked wingtips protruding out.
767 - Looks like a 777, but smaller and narrower. I believe some in DL's fleet have large winglets that stick way above the rest of the wing.
757 - Long, thin aircraft that has almost a pencil-like appearance. Sits high above the ground and has much bigger engines as compared to its narrowbody cousins in DL's fleet. If it's really, really long and thin, it's a 757-300. Can be difficult to distinguish these from the 767. Many different configurations out there, but most have two overwing exit rows, and all have two boarding doors forward of the wing (unlike any other DL narrowbody). Some with winglets, some without, so that's not really a deciding factor.
737-800 - Short, squatty aircraft with large winglets and a distinctive cockpit window pattern that narrows into small, rectangular slits. Two overwing exits.
737-700 - Visually similar to the 737-800, but shorter and has only one overwing exit.
A320 - Sits higher than a 737. Look for two small (very small) fins on the wingtips as opposed to a single large winglet. Two overwing exits.
A319 - Visually similar to the A320, but shorter and has only one overwing exit.
MD-90 - About the length of the 737-800, but with two large engines mounted at the rear of the aircraft that are almost as tall as the fuselage itself is.
MD-88 - Visually similar to the MD-90, but the engines in back are much smaller. Considerably louder than the MD-90. Has a badger-like tailcone that narrows to a vertically-flat edge in the rear.
DC-9-50 - Visually similar to the MD-88, but shorter. Tailcone narrows to a point instead of to a flat edge.
E-170/175 - The overall family is very distinctive, looking something like a mini-A320 with 737 winglets. Once you know what they look like, you'll never misidentify them again. No overwing exits on either model, but short of counting the windows or reading the nameplate when it's at the gate, I have yet to find a reliable way to identify the two variants.
ERJ-145 - Small, pointy aircraft with two rear-mounted engines that are very long as compared to the engines on their CRJ brethren. Mostly seen in the northeast, if I'm not mistaken.
CRJ-900 - Two large-ish rear-mounted engines, placed above the centerline of the fuselage. Shorter and narrower than a DC-9, with a more "pointy" appearance. Two overwing exits.
CRJ-700 - Visually very similar to the CRJ-900, but shorter and with only one overwing exit.
CRJ-100/200 - Shorter and squattier even than a CRJ-700, it looks like a glorified business jet converted for passenger use (oh wait...).
EMB-120 - I don't believe any of these are actually in DL livery anymore (all SkyWest). They only operate short hops out of SLC, and they are the only aircraft in DL's fleet with propellers.

If you're really struggling to identify an aircraft you pass by, you can always just walk up to the gate (or even an adjoining gate sometimes) and ask. The agent working the flight certainly knows what type of aircraft it is, and agents not working that flight can usually look out the window and tell you if they're not too busy.