Yup, small leaves are the quickest, easiest and most fool-proof way to identify Siberian Elm. Without the leaves... thick bark and a brownish-red to reddish-brown heartwood that is 2/3 to 3/4 the diameter. Normally really, really stringy to split (especially when green)... wait for the bark to separate from the rounds and a real cold (sub 0° is best) day this winter and the splitting will go much better. Siberian Elm is some really wet stuff, it holds a ton of moisture ('round 60-65% by weight) and that makes it seem like heavy, dense hardwood... but once dried it feels light compared to some others.
Red Elm has much larger, tougher and rougher leaves, thinner bark, the heartwood has less red "brilliance" (for lack of a better word) to it (it's more like an Indian brown), and the heartwood covers closer to 7/8's or more the diameter. Often Red Elm will split relativity easy green when compared to other elm, but it does get "tough" (not really stringy) as it seasons... I've split pretty large rounds of green Red Elm using just a maul, but the maul will often bounce on dry rounds from standing dead. Red Elm has a low moisture content ('round 45% by weight), about the same as Ash.