I use quite a few different websites to conduct my research while breaking down these cards. I thought it might be helpful to others to create a consolidated list because I don't use every site on every post. While I might have to do a Google search here or there to find other information, most of it is gathered from the websites below.
My first stop is usually GETTYIMAGES.COM to find the original source photo. To narrow down the photos, I usually type in the player name and the year I think the photo was taken. For these 2014 cards, that mean 2013. For Series 2 and/or Update cards, this may have to be adjusted as they sneak in some 2014 photos.
Some of the rookies will have a couple dozen photos to look through, while the veterans and stars will have a couple hundred. If I can use other clues from the card to determine the stadium or opposing team, I'll add them to the search to narrow it down more. You'd be surprised at how similar photos can be, specifically on generic motions (pitchers pitching, batters following through, etc). Sometimes you'll have to look even closer at things like the wrinkles in the jersey, the fans in background or the slight change in facial expressions. (If I remember to proofread this before posting, I'll add examples here. If not, call me out on it and I will.)
Once I find the right picture, the caption gives me the date of the game which I can use to find more information. Some of the photos are generic (as mentioned above), but if it can give me specific details about the play, I'll take it.
My next stop is almost always BASEBALL-REFERENCE.COM for the boxscore. This LINK will take you to 2013 boxscores, but you can change the year. While the caption might not tell me the exact play, Baseball Reference often has enough detailed information where I can use context clues. Not always, but a lot of times.
In addition to the boxscore for the game depicted, I use Baseball Reference to help fact check the Rookie Facts and blurbs on the backs of the cards. They also have rosters that help me with cameos if I don't know right away. Many of the non-card photos I use are screen shots from the site. Which reminds me, I'm going to get better about citing that.
From here, my path diverges quite a bit depending on the individual card. Sometimes, I'm still focusing on something in the photo. Other times, I've moved on to the back of the card. The rest of these sites are in no particular order, but I have used them all.
I'll admit my fandom is very Cubs-centric. Sometimes, for other teams, I can't tell the difference between a throwback and an alternate jersey so Chris Creamer's SPORTSLOGOS.NET has come in really handy for that distinction. Not only are they a great resource for my custom baseball cards for logos, but for last season, they added a jersey breakdown. This LINK will take you to the Cubs for 2013, but you can change teams and dates.
Before sportslogos.net, and still now occasionally, I would go to UNI-WATCH.COM for uniform information. Usually when I do a Google image search for a commemorative patch, a photo from this site would pop up.
If I can't get the statistical information from Baseball Reference that I'm looking for, I head over to FANGRAPHS.COM and try there. Some of the fact checking for Rookie Facts and blurbs is more easily found through here.
Occasionally, I'll do a Google search and reference articles, videos, stats or anecdotes from ESPN.COM or MLB.COM or WIKIPEDIA.ORG.
If I remember any other sites, or start using some others, I list them here as well. If you have any questions about researching a card, drop me a line and I'll do my best to help.