Starcraft – somehow check the supply of an ally in a team game

It sounds like you're building a large army and using it to gain map control, but then are falling prey to things like drops and backdooring.

If you have map control your first thought should never be: Crush my opponent, as you're essentially sacrificing an advantage (map control) for an unsure victory (because he doesn't control the map doesn't mean he can't take you). Instead your thought should be: take the map. I think you'll find this change in philosophy will lead to situations where you have a much more powerful economy than him and you're able to kill him with attrition.

Now if you're falling prey to things like BD here is a quick check list for you:

• Are you scouting? an Observer should be the first thing you build out of Robo. If you don't go Robo, a Hallucination works just as well. Sometimes scouting means sacrificing some Zealots in a pointless charge to just get inside his base.
• If you're scouting, are you seeing it coming? Overlords on the edge of your base, Starport with Reactor + 3 Rax, all of these are indicative of certain strategies you should watch out for.
• If you see it coming, how are you responding? If you have map control that means you should be able to respond quickly. Shutting down these back door attempts is often a bigger victory than a straight up conflict
• Finally, Do you have map control? If you don't have map control that means he does, and if he's beating you with map control then your problem is not "how do I deal with a base trade?" but rather, how do I regain the advantage.

Quoted from my answer to How do workers work when I gain control of them?:

When you get control of someone's units because they've left the game, you can command their units and buildings as you like (attack, lift off, change rally points, use energy, etc), and you can also spend minerals and gas. You can build buildings, train troops, etc. The resources come out of your bank, but this is because when the ally leaves, their resources are split among remaining allies, and their further income gets split among you too.

Although you are spending "your" resources, the buildings and troops remain that player's for gameplay purposes. Pylons will make energy fields for them, not you. Depots will raise their supply cap, not yours (and they can be supply capped even if you're not).

Applications:

• If an ally drops early, make sure to send their workers harvesting, and train more. It will cost you some minerals, but you will get lots back from the split harvest.
• If an ally drops late, you can basically take over for them, building bases and troops and waging war. At the very least, you can add their remaining troops to your army.