The liquid yeast makers list alcohol tolerances on their websites. You should make a starter for most beers over 1.065 I believe. A lot of people use Mr. Malty from what I have seen. I typically just wing it.
Even if you under pitch, your beer will ferment, just not as quickly and with more by-products. I think it also stresses the yeast so they might break down faster once they settle. I never leave a beer in primary for more than a month though, so I don't tend to worry about it.