How to tell if it's a hybrid?

Look for a mixture of shapes and colors.

For instance, on your second one it's got a pretty round body. I'm not familiar with your area and am not sure what all species you've got, but I believe you would have quite a few bluegill and pumpkinseed and the round body shape would indicate one of those. However, when you look at the coloration and the edging on the fins, it's not congruous with either one of those species but are more synonomous with green sunfish. With a lack of opercular markings to indicate pumpkinseed, I would call your second one a bluegill/green sunfish hybrid. It's got the body of a bluegill and the fins and coloration of a green.

I'm no expert and don't know what your other two are but I'm pretty sure they're pumpkinseed hybrids of some kind.

It comes down to knowing what the characteristics are for each species. Let's walk down the three:

1. Green Sunfish x Pumpkinseed

 - Pumpkinseed spotting and head/mouth shape (Green Sunfish mouths are more terminal and wide vs Bluegill and P-seed)

 - Orange fin highlighting on the anal and pectoral fins - the only sunfish you'll catch around here (I think) with those are Green Sunfish (primary identifier in this case)

2. Green/Pumpki/Bluegill hybrid

 - Green sunfish fin coloration

 - Green sunfish mouth/head shape (kinda - this alone would make me think twice but not come to a conclusion. Definitely helps indicate in this case, though)

 - Pumpkinseed spotting (extra-faded, which can happen in hybrids, especially of more than two species)

 - Bluegill body shape (round with 'shoulders' vs narrow (Green) or straight up round (pumpkin)

 - Bluegill opercle (no red dot/highlight)

 - Bluegill throat? Almost looks orange, like a male bluegill.

3. Pumkinseed/Green Sunfish

 - Pumpkinseed spotting. Sometimes Green sunfish have similar spots, but it usually isn't quite as quilt-like.

 -  Pumpkinseed mouth (not forward-facing)

 - Green Sunfish fin highlighting

The usual dead giveaways to me are the fin highlights (Green), spotting (Pumpkin), and opercle/shoulders (Bluegill).

If anyone wants to disagree w/my IDs or anything I've said, please do. Sunfish aren't really a forte of mine, but after catching approximately 50,000 small Greens and Pseeds during the contest fishing for other species, I'm starting to get a handle on it.

I stand corrected, didn't see the pumpkinseed in that one.

Most hybrid identification comes from a particular fish sharing traits of two different species. Sunish are a little tricky because they interbreed like crazy. Basically if you can't tell it apart, because it has traits of two species, it is probably a hybrid. Some are extremely hard to tell their parental lineage, and if I ever really needed to know.... I'd bug some DNA people. 

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I am pretty sure shoulders refers to the bumps above and below the head that big bluegills get. Almost makes the head look seperate from the body. 

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Some things I use for Southern Ontario:

Bluegills opercular flap (the black thing at the end of the gill flap) is completely black  and they usually have lateral bars on their sides. I find these the easiest to ID. They also have black . They have small mouths and black spots on their anal fin and second dorsal (share this with green sunfish)

Green Sunfish have large mouths, yellow-tipped fins, they are more elongate (like bass compared to the frying pan shape of the other sunfish), they have blue dotted horizontal lines along the body (much smaller than colours on pumpkinseeds and longears). They also have a dark Black spot on their anal fin and second dorsal fin.  They also have a pale border around the opercular flap, usually has a very faint bit of red to it, though not as prommminent as it is on pumpkinseeds. 

Pumpkinsseds have small mouths, a pale border around the outside of the opercular flap with a red mark on it. They usually have somewhat large orange spots on their sides which gives them their name pumpkinseed. 

I am not good with longear sunfish and my ID books are 1800 km away :(. I have yet to catch one in Southern Ontario, but if you find them feel free to let me know :). 

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I think your first and third fish are pumpkinseed greens, opercular flap  and mouth matches pumkinseed, and it lacks the black spots on the dorsal and anal fin. But the yellow fins, face markings (blue stuff on cheeks) and the body colour (for the top one) match green sunfish

Your second one looks like bluegill greeny to me, mouth of bluegill and body colour of one, but everything else looks green sunfishy. I can't zoom it so cannot see any of the pumpkinseed marks.

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A really good book for Ontario is the ROM field guide to Freshwater Fishes of Ontario. Can get it pretty cheap (around $30, I got mine for $20 on sale) It doesn't include Northern Sunfish though. It even includes the lateral line scale counts and fin ray counting if you really want to have fun (this is unfortunately necessary if you decide to soak your fish in ethanol for 200 years)

Basically go through the characteristics and see how many line up with a particular species and how many with another. A taxonomic key is best, but they are a bit harder to come by. The one in Freshwater Fishes of Canada by Scott and Crossman is fantastic with the taxonomic keys, but it's really hard to get ahold of since it was published in the 70s and hasn't been re-released. Oh, and don't worry about all the characteristics matching a particular species, there is a lot of interpopulation variation within a species, so if your fish has like 90% of the characeristics of a species but the lateral line count is off, don't worry about it. 

Those are some cool-ass hybrids. Predominantly GreenGill, but they both have P-seed in them (many generations back - the spots are super faded, but in my opinion definitely there).

Super legit fish.