Is there any evidence that James (Prongs), Sirius (Padfoot), Lupin (Moony) or Peter (Wormtail) knew that Snape loved Lily?
TL;DR - maybe they suspected he fancied her, but that's about it
EDIT - this is all pretty much speculation besides the Lupin quote, but I imagine that's all we can do, since this is a question based largely on speculative subjects.
Lupin didn't appear to know, given his position in the discussion about trusting Snape. He could have been lying, but that's a bit of a leap.
"It isn't our business to know," said Lupin unexpectedly. He had turned his back on the fire now, and faced Harry across Mr Weasley. "It's Dumbledore's business. Dumbledore trusts Severus, and that ought to be good enough for all of us."
"But," said Harry, "just say - just say Dumbledore's wrong about Snape -"
"People have said it, many times. It comes down to whether or not you trust Dumbledore's judgement. I do; therefore, I trust Severus."
Besides this, given the immaturity of the Marauders at the time - especially Sirius and James - it seems unlikely they would have paid much attention to the relationships of others. We know James liked Lily, but it doesn't seem that he really knew her (don't have a quote right now but he refers to her as "Evans", I believe). Lupin spent his time keeping his head down, and given his nature I doubt he would have brought it up if he did suspect something. Pettigrew...well, 'nuff said.
James may have had an inkling, which could have contributed to his bullying and rivalry of Snape (though we must remember, he literally bullied Snape from the first moments, so who knows?).
It's also unclear how much time they (Snape and Lily) spent together in Hogwarts, since the Slytherin/Griffindor divide is historically a big one and Voldemort was on the rise, fuelling tension between purebloods and muggleborns. It's possible that the majority of their time together was spent during the summer holidays, or in secret, since it is rather bizarre that no-one ever tells Harry that Lily and Snape were best friends.
Finally, Lily and Snape were fifteen when they had their fight. "Love" can be, and usually is, a very different thing at that age. If the Marauders did did suspect they'd either class it as a friendship, or puppy love, especially when thinking about it in later years. It's practically certain they didn't know the real depth of his feelings, because, as we all know, no-one ever figured out why Dumbledore trusted Snape, or why Snape stopped being a Death Eater.
What I'd be interested to know is how Lily spoke of Snape in later years, especially to James. Though I suppose since he was a Death Eater at the time her opinion would be much the same...