Professor Oak will eventually ask you to find six Pokemon signs for him. There is one sign in each course (except Rainbow Cloud).
Beach Course - Kingler Rock: Near the beginning of the course, look to your left, there will be a moss covered rock that looks like a Kingler.
Tunnel Course - Pinsir Shadow: You must release Zapdos from it's egg then look to the right near the end of the course. There will be a shadow that resembles a Pinsir.
Volcano Course - Koffing Smoke: Near the beginning of the course you will see a purple puff of smoke come up to your left. Throw a pesterball down there and the smoke will resemble a Koffing.
River Course - Cubone Tree: You must use the PokeFlute to awaken the sleeping Vileplume. Once Vileplume's Stun Spore has cleared you will see a large tree that looks like Cubone.
Cave Course - Mewtwo Constellation: ABout half way through the course you will see about 6 or 7 purplish-white stars to your left. Take a photograph and it will come up as Mewtwo.
Valley Course - Dugtrio Mountain: Straight ahead of you when the course starts you will see the huge mountain that looks like Dugtrio.