Sometimes it's hard to tell from just the dipstick alone. Drain your oil completely into a glass or plastic see-through container. Wait overnight and check it in the morning. If there is coolant in it, it will separate from the oil and will "float" to the top. You won't miss it if there's water or coolant in there. You can also do this with a non see-through container, but it's harder.

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280