What to do when you suspect players of cheating? Options

[Lady Door]

During the past few games that I have had the opportunity to run I have noticed an increasing amount of amazing rolls among a few specific players in my group. Now, I don't know wether or not this is because I'm new to GM'ing and they don't think I'll catch on or wether they just don't want their characters to die. But it's really starting to get irksome...for example, I had this specific player up against a Nosferatu. It was a melee combat situation and somehow at the end of it all the character walked away without a scratch. Now, I would be inclined to just explain this as incredibly lucky dice rolling or good combat sense if this didn't seem to happen EVERYTIME! Also, this player seems to be rubbing off on another player in the group so having a fair and balanced game seems to go right out the window. I don't want to throw the group in an incredibly hard fight just to even the score because the rest of the players in the group are very honest about this sort of thing.
Just FYI, our table is not big enough to allow our entire group to play tabletop so we usually just sprawl throughout the living room. Does anyone here have any ideas or advice on how to help curb players "padding" the dice without alienating the players?