In general, the best way to learn to read to a crowd is to play as often as you can. This may involve taking your mobile DJ equipment to some concerts you wouldn't necessarily want to play, such as weddings. Collective reading is the most transferable skill a DJ can acquire between different types of concerts. To read to the crowd, you have to be on the same level as the crowd.
Watch them as you raise your hands and they'll respond by doing the same. The feedback you get from the crowd is exactly what you should be aware of. Play the song, observe their body language and facial expressions. If you see faces moving up and down following the rhythm of the tracks, then you're doing well.
Even when you've done everything you've learned about DJing and the audience isn't responding, then it's better to stop trying too hard and assume that the audience is like that. Learning different things to say to get the audience excited before the concert can be helpful, but don't overdo it. It seems easier to know what the public wants to hear at parties, because they just write a request and I know most of the people who are there. Once you have mastered the skill of reading to the crowd, you should realize that the crowd also reads you.
If you have access to a microphone, use it to your advantage and tell the crowd a few words per song. So what do you do? Well, as we've already said, you have to try different songs while reading to the crowd to get the desired reaction. The audience can give you their opinion on a specific song, but you'll have to choose the exact song you want to play. This is something that has happened to me, and I have even wondered if I really knew how to interpret the crowd.
Playing the perfect set for the perfect audience is something that will happen to DJs who go the extra mile. Whether you're playing at a nightclub, a bar, a festival, or as a mobile DJ, knowing how to read to a crowd is one of the key skills needed for a successful set. Learning all of the tips above and paying special attention to the 6 bullet points above will give you a big head start. They fail because they haven't played the right music, because the audience didn't feel it, and because they didn't know how to interpret the crowd and change it.