When you see that the opponent has nothing in your base = cheese such as cannon rush, proxy barrack/gateway but normally its too late and trying to survive might be hard.

overlord can spit a spy rite? i was shocked when i watched replay the guy put a marine in my base. No wonder all my banshee taken out by those pores although i hid them at far corner

Usually you would notice the changeling when you try to move your army and you notice that got one unit not moving.

Usually if want to sneak a changeling in, you would have to sneak it at a spot they won't notice. Not easy though if they put up turrets around the base.

But later they move the army that time also they'll start to notice. Why that marine not moving one haha.

i have a big problem scouting between rushing and teching opponents, especially zerg and terran.

terran cos my probe sure die after his first rax/marine, then after that i'm blind. so how would i know whether he's turtling and teching, or pumping out mass mmm from 4 rax?

zerg i got trouble too cos e.g., if scouting toss i see 4 gate i know he'll come at me with mass gateway units. but zerg only need 1 pool, and with queens and fast expand he can easily come with me with 30+ lings in like ... 1 minute?

Usually you would notice the changeling when you try to move your army and you notice that got one unit not moving.
Usually if want to sneak a changeling in, you would have to sneak it at a spot they won't notice. Not easy though if they put up turrets around the base.
But later they move the army that time also they'll start to notice. Why that marine not moving one haha.
Oh yea. Can just set it to follow one of the troops.

A few things to be vary of for various races.
The hard part is still scouting the terran as they wall off.
Worst case would be to sacrifice 1 overlord hovering nearby just to see what structures is he going.
This is usually for cases that are not cheese since it was already covered above.

Terran:
3-4 barracks with tech labs.
Incoming marines and marauders on the first attack or only marauders.
Concussive shell and stimpak is probably on its way.

Barracks with a reactor and you managed to see a factory.
Watch out for hellions. Try to block your ramp with units and move your spine crawlers to it when creep reaches there.
Best is to just get speed for your lings to counter this.

He is going 1/1/1. 1 Barrack, 1 Factory and 1 Starport.
Mech build incoming. Mostly marauders, tanks and thors with vikings and medivac.
I seldom meet this as protoss though. Should usually happen against zerg players.

Zerg:
A roach warren after the pool and hatchery still not upgraded to lair.
Incoming roach with ling rush.

Lair almost immediately and there's no roaches.
It's either hydralisk or mutalisk. Watch out for which building comes out first. The hydralisk den or the spire.

Protoss:
2 Gates up before the cybernetics core.
Watch out for a zealot rush. Usually next you see is another 2 gates coming up.

2 gates and then a cybernetics core. He is going the normal zealot, sentry and stalkers.

1-2 stargates
Phoenix or void rays are coming.

robo facility with 2 gateways
Immortals and probably colossus is coming. Watch if he builds the robo bay.
Hard to scout this out though as it is already late game and there is probably a lot of stalkers to shoot down your scouting overlord.

Hope I got those right.
Please add or correct me if I'm wrong.

Haha that's a really bad tactic to use against zerg especially when you can pool early and nothing he can do to stop your zerglings swarming him down.
He was really asking for it. It works best against protoss and nice also against terran. Even though terran can lift and land somewhere else, the protoss player already denied his economy for that amount of time to lift and land.

You can also just keep going lings and rushing into his base and destroy everything.
The ling rush part happened to me. Could not really contain him and I got overwhelmed later in the game as he kept harassing my expansion when we were both really low on resources. The ling part was actually at the beginning and at the end. He did go hydralisk in the middle but I think he gave up on it after meeting my phoenix army. If you want I can post the replay for you to see how he played with his speedlings. He was quite good at using them to harass non-stop.

See what happened when you are not scouting, he is quite a mediocre player, problem is that I am worse...

Dont laugh at me lol