Generally you need to measure two things (as a minimum):
1) What proportion of Leads from a specific source are qualified, versus unqualified (don't forget this second bit). This gives an idea of the quality of this source. For example, if you use the old "business cards in a goldfish bowl" approach, you might find this ratio is quite low. If you only take down details of delegates who actually talked to you at your stand, it is likely to be higher.
If you go to 2 shows, and from one you get 100 qualified leads, and 50 disqualified, and from another show you get 90 qualified and only 5 disqualified, which is more valuable to you? the amount of time wasted disqualifying the 50 might be a significant cost. (PS: there is no right answer to this question, you need to figure out the metrics that work for the success of your business).
2) Of the qualified Leads (that you convert to Opportunities), what proportion turn into wins, and at what value. Finer detail of this will include things like how quickly those turn around - perhaps from one show you get fewer Leads, but a higher proportion qualify, and they turn into deals in three months. Another show might generate more Leads, but a lower percentage qualify, but of those that do, more turn into deals of higher value, even though they take longer to close.
Again, this gets quite complex. Are you all about the eventual revenue numbers? Or does the time and effort to get from qualify to close have a high cost and make a difference to the bottom line? Or do you know that historically in your business, bigger deals tend towards stronger long-term customer relationships and higher profits? Are 2 deals for $100,000 worth more in the long run than 10 deals for $20,000?
You will want to measure something like actual revenue by Lead source (you might use a campaign for each show, for example). If you can break this down by revenue generated after one month (or quarter, depending on your sales cycles), two months etc this is also useful.
Different shows might attract customers who are already at a different stage of their sales cycle. Some will be ready to buy, others just starting to think about what you do (or have never even heard of it until now). Some shows attract customers of different sizes, sectors etc. Some shows are not attended by as many of your competitors... etc.
Simplistically, the big question is "did I raise more revenue (or margin if you can measure that) than it cost to go to the event in the first place?" There are built-in sales reports to measure these sorts of outcomes of sales vs original source campaign. You might be able to use these as a starting point.
If marketing budgets are tight (aren't they always?) you might need to decide which of three events you go to again next year, and which one to drop.
Another metric I have seen people look at is to compare the ratio of eventual wins/losses of qualified opportunities relative to who qualified them. For example, Alice qualifies 10 Leads out of 20, and then 5 of the 10 end up as wins. Bob qualifies only 7 Leads out of 20 (from the same source), and then 5 out of the 7 end up as wins. Alice is passing on 3 more Leads that cost time and effort to follow up as Opportunities to no avail. Bob's eventual outcome might have been the same, but less sales effort was wasted, which is more profitable and less frustrating for the sale team. This metric can then be used to go back and look at process, how decisions are made, and retrain where necessary - maybe Alice be more discriminating, or maybe Bob should be less fussy: could one of his disqualified ones have also turned into a sale? Of course, with small numbers these things are hard to compare scientifically, but over time, with larger volumes, they are more robust.
Measuring things like sales velocity gets more complex because usually you need to do some other calculations (especially date differences) before you can analyse the data. You can either do this with things like calculated fields, workflows or plugins; or third-party tools like North52 formula manager (they actually have some tools specifically to help with sales velocity stuff); or during the analysis phase using Analysis Services, or a reporting tool like Zap BI that enables these sorts of pre-calculations (using SSAS under the hood, but with a user-friendly web-based GUI).
