Look at their traffic report and find out how members found the community. Whether they are ranked well on Google search. If not, are you able to boost site visibility on search engines? How active are their members and how many are there? Are the moderators and owner willing to stay with you? Also things like how can you contribute to improving the community? Will you be doing it as a hobby or for the money? Depends on what your priorities are.
It definitely depends on your priorities as Jeffin rightly says, but whatever reason you have for buying it you need to do your research. If it's for financial reasons then look at their traffic results, traffic sources, monthly revenue, revenue sources, and whether you can improve each of those aspects to increase it. I'd also look at how big the community is and its growth rate. You might want to think about whether the owner will stay because it might well be a niche that you know nothing about, and I personally don't think owning a forum where you're clueless around its core subject is beneficial for anyone.
It takes a lot of planning but the potential reward is there.
I wouldn't base their google rankings as the reason why they should or should not buy the site. That can easily be changed if you work hard at it. I'm ranked #1 for CODForums or related keywords (likewise their ranks are different depending on their keyword), and this depends on where you live or if you visit'ed the site before. When I searched CODForums.com back then, it was ranking at - at least #3 or #5 on the first page.
If this is monetary gain, you can either look at their traffic stats, or understand the market very well. If you know there's a huge potential for niche community (like a Call of Duty community/forum), then take that risk and don't look back. If it fails, it fails. If it doesn't, and its taking a long time to get where you want to go... Do it. Work at it until your goal is reached.
Let me put it in another way so you understand what I meant. Marvel Vs Capcom 3 was about to come out, and someone was selling several MVC3 domains, I jumped on it and paid only $40 - which is not much. But the guy originally bought it for $10 (and 3 other $10 domains on their own) and a vBulletin license - which was not included - I already had an xenForo license and could merge the site into xenForo. I paid for the domains, and database. A month later, the traffic exploded, 3 months passed, I was able to ride a tidal wave of visits and growth. This wave went on until June, after which the traffic slowly waned, slowly teetering off, until the fall when Capcom announced the update, which ultimately killed off any more interest in the actual game. So, sites like mine suffered. After a while testing the traffic numbers, I bought 2 more domains that would be like a general purpose Marvel Vs Capcom site. That didn't work. So, now I'm considering merging that pre-existing xenForo into another one of my sites. Always have a plan B ready in wings.
With CODForums, I came upon my own research in my time going to communities over the years. I saw a community that was around 50k members, and another one that is thriving to 14k members, I saw opportunity. So it just happened that I stumbled across codforums.com and I became administrator for a while until I told the owner: I want to buy this. Even luckier, I had funds coming, and I was able to cement the deal. Took me a while to get the whole thing done - In the fall, I paid $500, then in the winter, I paid $500 - a combined $1000, plus $101 for getting all the networked domains moved over to my account. At first, it was on vB3, upgraded to vB4, didn't go well - and you know why. vB4 was a crappy software with crappy SEO, you know the story. I transferred to xenForo that august, and the growth was slow, very slow. But I wasn't discouraged. There's always friggin' next year. Black Ops 2 came out, I bought a new skin from my bootstrapped $100 from google, and traffic exploded that same day Black Ops 2 came out. Next day, my server went offline. With CODGhosts, the traffic was almost the same, except that I moved my server to a new one due to Multiplayer reveal (august) taking my site offline. This time, the launch traffic did only 200 less than BO2. Thanks, Ghosts. Thanks for nothing.
Fortunately, I have 3 different plans ready to take this to a whole new level.
Moral of the story; don't be discouraged by the numbers at first. Keep at it until you are satisfied or better.
Especially if you have a marketing plan, and/or is an expert at marketing.