Is there any way to tell which type of brake fluid (glycol or synthetic) is already in the reservoir? The handbook says use dot 4, but on looking for this I found that some dot 4s are described as synthetic while others are not and I have seen warnings on these forums about not mixing the two.

DOT 4 and 5.1 are polyethylene glycol based. DOT 5 is silicone-based and does not mix with the polyethylene glycol based fluids. All brake fluid is synthetic, putting 'synthetic' on the bottle is just a marketing ploy.