Determine your initial qualification criteria. These could be a position in marketing within a large technology company. Then, determine your ideal prospect profile. This would go beyond the initial criteria to include companies who outsource services in your category and who have a current need. You need to decide up front what you're looking for and train your team to go after only those prospects.
Capture lead qualification information.
There are a number of ways to do this, all with their advantages and disadvantages. Some firms use badges to scan, using tools provided by the trade show sponsor. Others have their own qualification forms or kiosks that they use. Some collect simple business cards and write on the back. All of these work if you have the right follow-up process.