If there's a check valve, it should be on the hot water side; not sure you'll see it, as I think some are built into the hose fitting.
I'd say, close the cold water valve, open the bypass valve and go for it. Probability of the check valve leaking by is low, IMHO.
If you're concerned, when you're done winterizing remove the anode to the HWH and check for antifreeze. If you find any, just flush out the HWH through the anode/drain hole.
1988 Coleman Sequoia - popup (1987-2009) - outlasted 3 Dodge Grand Caravans!
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado 1500 Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10