I've got a table like web page and on only one column you can hover over the rows and tooltips appear. At the top of the column I've got a little info symbol ('i' in a blue circle) but I don't think it's intuitive enough. What is a decent way to indicate that tooltips are possible?
A tooltip is a helpful thing, but should not be used to display information that might be essential. If the information is additional rather than essential I believe your approach is pretty decent.
Let's have a look at someone else who did it exactly like you did:
This is taken from the Audi website, where you can choose and configure you car. Some options such as some types of mirrors need further explanation in case you don't understand the difference and have the little "i" icon next to them, as shown on the picture above. The user can hover on the i icon and then gets to see additional information:
I'm a fan of using a dashed underline to indicate a tooltip. I could see where it may be too clunky-looking in a table, though.
You could change the color or style(italics, underlined, shadowed,..) of particular line for which you want to show tooltip.
Follow that style uniformly across all text/object in your site which has tooltip.
Some days ago I've registered at slack.com. And they've implemented rather interesting solution.
At first users see animated visual cues which pop out due to animation - it's hard to skip them:
And after click on the cue an user sees the information box: