Spanish mackerel are similar in appearance to small King mackerel and Cero mackerel.
 All three are very similar in shape and coloration. They may be distinguished as follows:

The lateral line on Spanish and Cero mackerel slopes gradually from the top edge of the gill to the tail.
In contrast that of the king mackerel takes an abrupt drop at mid-body.

The first (spiny) dorsal on Spanish and Cero mackerel has a prominent black patch.
The King mackerel has none.
 As all three species normally keep the first dorsal folded back in a body groove,
this difference is not immediately evident.

Spanish mackerel have prominent yellow spots on the flanks at all sizes.
 In addition to such spots, Cero mackerel have one or more yellow stripes along the centerline.
Young King mackerel have similar, but slightly smaller spots;
these fade away on individuals weighing over 10 pounds
 but they may still be seen as spots of slightly darker green on the upper back from some angles of view.

Trolling in more detail

Spanish are very well-known for taking lures.
So unlike most fish I often tell people to troll lures they don't use any more or don't like much,
The lures need to be smaller 3 inches or less. and while it is thought that the Mackerel will bite through the line,
which is sort of true it is the buddies of the one you have hooked which bite through the leader/ line.
When one Mackerel gets hooked up its excited thrashing around makes the rest of the School think it has the best food ever,
and they come to get in on the feast. Mackerel will hit anything which moves or makes noise,
which if you think about it, if a mackerel got 4 foot I don't think I would go in the water.
Getting back to the point the one hooked gets the other excited and they start darting about looking for the thing which got the other fish so excited and your swivel or the knot which attached the leader to your line is a perfect target even where your line is hitting the water,
 the V it makes while you reel in is fair game to crazy feeding Spanish.

If you are looking for very good lures to troll,
 try the Maverick Golden eye in the small version or the Storm twitch these lure have a lip on them and do not spin when you troll them,
use a light rod. Lets say a 6 to 12 pound class 7 foot spinning or bait cast set up with some thing 6 pound test or more.
I troll at about 3 miles per hour for the mackerel.

Another trick I learned is to keep the rods I am trolling in front of me,
 I mounted Rod holders on the front deck of my boat so I can watch where I am going and see when a fish hits.
If you do not have front mounted Rod holder any couple of your friends will do just fine.
 If they are in front of you, you can see when the rod gets hit and stop the boat if the people or Rods are behind you,
you may not find out about what is happening until you are out of line.

To make sure the rods and lures are working right watch the tip of your rods.
They should be vibrating the tip of the Rod that is.
The lures have a lip on the front of them which when pulled through the water causes them to be pulled under and the force of the water gets deflected off of the lip causing the lure to swing side to side imitating a swimming action of a bait fish. that Side to side movement of your lure, makes the tip of your rod bounce ./ vibrate in time with the side to side action and So when you look at your rod it should be bent a little and vibrating, if the rod end is bouncing or Best case pulling hard, meaning joy of joys you have a fish on, or if pulling down a bit then rebounding back up would mean a very small fish or a weed has been caught in the lure. either way reel in clear the lure and start again.

If you are going to cast for mackerel I would suggest Spoons or Got-cha lures. Spoons should be small Like the Clark Squid Spoon #0 or #00 problem is they are too light to cast so I put a water fill-able casting bubble on the line a head of the Spoon to give it weight in the air but once the bubble hits the water it is nothing just a plastic bubble full of water. The got-Cha lure is different it is a jig type bait and should be reeled in moderately fast with a jigging action, which means while reeling steady you would sharply lift the tip of your rod. Then lower it, repeating this the whole time you are retrieving the lures

Spanish Mackerel are not a fish to catch and freeze for later, The meat is not tasty after being frozen. So I only keep them if I am going to have time to clean and cook them that night. If I am going to have time I will keep 3 for two people, The seem like more than you would want but if you have a grill some butter, and McCormick brand lemon herb Spice. It may not be enough. I butter one side of the Fillet, then sprinkle some of the lemon Herb on it and place that side down on the hot grill. Oh yes when you fillet a Spanish watch for bones by the belly, Spanish are tricky to fillet properly and get out all of the bones. after removing the skin rub your fingers up and down the meat. Your finger can feel bones way better than your eyes can see them. When the side facing down on the grill is brown I butter the top side add some seasoning and fill it over. brown on both side is done.
Serve with your choice of side dish as you will only be thinking about eating more of the mackerel any way.
Good luck out there, have fin and be safe
Fishin Frank