Preface: I do it sounds like the exact same work as you, but don't follow any sort of "SMART" protocol. My boss is also very understanding that some endeavors will work and some won't, if anything he is more inclined to take risks on some marketing ideas than I am.
I'm in charge of designing and developing our main web site, creating email templates, increasing social engagement and visitors, building web graphics, trade show collateral, product data sheets, webinar graphics, etc
Social Engagement and Visitors
Well this is easy enough. Did you see an increase in Likes, Visitors, etc? Did changing the design result in longer visits? If you have a "Goal" did the change see an increase in users reaching that goal? A lot of times websites will talk about setting Goals only around Checkout but as an example we only sell to Dealers. The ultimate Goal of our website is an End User sees it, uses our Dealer Locator, and then clicks through to the Dealer's website. That's our Goal. Design for that.
Product Data Sheets and Trade Show Collateral
How is it received by your target market? Being Data Sheets it sounds B2B, does your company have any long time loyal clients? Talk to them about it. What parts of the design do they like, what do they find confusing, what do they struggle with, etc. For example we sell items in 56 different color combinations and had a very confusing color matrix that my boss was originally adamant about. After talking to Dealers though none of them were using it because they, like me, found it very busy and confusing. On our new catalog I removed it entirely and instead show a small sample of actual products in different colors fanned out.
As far as making something like this measurable it is much more tricky. First you have to have historic data which you may not have. But let's say you do --- you can look at how many catalogs did you give out? How many leads did you generate? Or how many actual sales came through? Or all 3 even. Part of this depends on product and lifecycle. For us if we go to a show and gave out 30 catalogs last year, and go again the next year with a more targeted catalog and end up giving away 100 then that's a very real success. It's one we're currently experiencing as I continue to create different marketing materials depending on the audience it'll be given too.
Website Graphics and Webinars
This can get a bit tricky as well (really all of this is). We don't create webinars here but we do have a technical product that requires some advanced knowledge. One thing we look at is a Content Drilldown to see where visitors are spending their time, then I talk to my coworkers that generally field calls and see what people are asking about. As an example we have a "Sticker" page that talks about the Stickers construction and benefits. But it was receiving a disproportionately short time on page compared to other parts of our website. Talking to coworkers I found they're calling in and asking some key questions - how to order more stickers and how to customize the stickers just as an example. So I created new graphics, new copy, and laid the page out in a way that "How to order" was very prominent. Time on page increased, calls asking about it decreased.
While we don't do webinars I do develop videos for us. One of the most successful things is when a dealer calls to say how much he likes the video and used it in a presentation with a client to sell our product. Another time a Dealer gave the video to a client, the client put it on their facebook page and we saw our largest ever traffic spike. That video is clearly successful in delivering the message it was intended to. (It's about our product's security)
As far as specifics related to SMART goes I don't think I can answer that. For one thing Product Lifecycle will be key. Our product has a 10 year lifecycle generally. We don't go to tradeshows expecting to sell much. We just want them to know about us so when they do upgrade their system we'll be something they consider. So a realistic goal for us might be if we spend $10,000 on marketing to Universities (trade publications and conferences) that we see an increase in calls and/or quotes related to Universities over the next 3 years. I'm not even saying sales just calls/quotes. That tells me our marketing during that time period to the University industry was working. But this is just a very rough example and yours will be completely different.
Hope some of this helps you.