I've been researching agents until my file is bulging and arranged by best fit to long shot, but there is still something eluding my research, and that is the Agency itself.
A good many are venerable and thus, able to be researched. Should the happy day arrive when I find representation, I would feel relatively certain the Agency or its agents won't be imploding the day after the ink dries. This is not the case with new(er) agencies, agencies that post elevator music with their opening page (please don't) or where little or no information is available. There's a surprisingly large amount of that.
As a venerable agent with a well-established Agency, you see from the inside of your industry. Can you offer a bit of guidance where Agencies themselves remain a mystery.
Since anyone can hang out a shingle and call themselves an agent (or agency) you're wise to understand that some vetting is very much in order.
BUT, I caution you about assuming all venerable agencies have a spiffy electronic footprint. That is not always the case. Some very good agencies have a very modest presence on the web and you'd be foolish to disdain them for that reason.
Look at who they represent. Google the authors and books. Look at who publishes those books.
The best way to distinguish between an established agent /agency who has a terrible electronic presence and a new agent/agency who has such limited experience you might think twice about querying them: how they talk about queries.
If an agency has almost no information about sales, and no address posted, and isn't trying to entice you to query with statements about how they love authors, you've probably run in to an agent that simply isn't looking for new clients. (Phoebe Larmore comes to mind here)
If an agency has no information about sales, no address listed and is chockablock FULL of ways to query, instructions on querying, and essays on how much they love authors, you've run in to an agency that probably doesn't have a lot of sales.
In general you want an agency that's been around for a while. My preference is for agencies that are not sole proprietors but many very good agents have solo practices.
Remember, agenting is not an entry level job.
If you're evaluating an agency, look at where the agents started their publishing career. Look at how long they have been in publishing.
If that kind of information is not on their otherwise chatty website, it's fair to assume they don't want you to know cause it's not reassuring info. In other words, they didn't have a previous job, or they've been in publishing for five minutes.
There's almost no way to fireproof yourself from an agent or agency imploding but you can certainly ask some questions that might give you a heads up.