1990 GMC C1500 White Smoke Coming From Exhaust Pipes

Hello I was just wondering if anyone could help me on my problem I have. I bought this truck a 1990 GMC K1500 for $1500 and it has a lot of miles on it of course. Well the guy said it smoked and he was told it probably needs value seals. White smoke comes from exhuast pipes when I crank it up and when I get on it. It does this anytime of the day or in anytype of weather. Coolant level seems to be doing fine. Doesn't run hot at all. Doesn't smoke driving around. Only when I crank it up, rev it up, or get on it. Can someone help me out please so I know what to fix. I don't want to take it to a auto shop and them give me the run around.

Thanks,
Tommy