How to pick a good investment sales team?

Things I would look at / vet:

1. Deal flow - how many deals a year are they doing? Average size per deal?
2. Nature of the deals they are working on? Are they pretty institutional or are they dealing with more mom and pop groups? You can make money doing both, but the formal is a more durable source of income and future deal volume. Are the deals intown, suburban, tertiary markets that you're driving to, etc? The beauty of being a young person in investment sales is that you see deals up an down the risk spectrum, in lots of property types, in varying locations and those deals are being pursued by different types of capital with differing levels of sophistication. That enables you to look at the universe of buyers/potential future employers and decide who you think is smart, who is an asshole, who has money, who has to go raise money every deal, who causes problems, who fucks around and doesn't close, and who brokers like. All of those experiences are super valuable (more valuable than the pittance you'll get paid... initially) and will help you immensely if/when you decide to bail our of brokerage.
3. Background of the brokers - have they always been brokers (this can be good or bad it depends)? have they spent time on the principal side? You can sometimes gauge how sophisticated they are based on their background.
4. Caliber of the folks on their team - I'd be looking for upper tier regional state schools and good to great private schools, you will inherit some of these people's network via osmosis if they like you
5. Brokerage flag - all shops are different in all markets, but don't underestimate the power of a good brand early in your career. Their are tiers here and anyone who says otherwise is full of shit.

That's just a few things... there is more stuff I could add, but I'll let other folks chime in. I think there is a ton of value in starting in investment sales... if you start with the right folks. Avoid the two-bit hucksters from Marcus & Millichap.