Can you actually determine the mAh of a battery? You can skip the story part but for better reference, please read it:

A Little Story

My phone claims it has a battery of 4000 mAh that lasts for 3 days. I got the phone, really psyched because of the battery. When I bought it it has 50% left on the battery. I did some texts and some first time testing on the phone, but no heavy usage. The phone lasted 19 hours before saying it was now low on battery (5%). 

The next day, I charged it for six hours, turned on the 3G and used the Facebook app, Opera Mini, and Google Play for four hours before I removed the 3G. The battery read 29%. I was disappointed. Most of the people who bought this phone reported that it should last 10 hours with 3G on and heavy usage. Most people even say that with heavy usage their phone lasts 2 days, using the internet, apps that require it, and gaming with the phone. 

Now I'm on the process of claiming the warranty and I called some service centers to determine where I will take it to. The first service center claimed that all Android phones last for only one day, the second one claimed that even if only a Messaging app is on, if I leave it open even if my screen is off the battery will be drained quickly. The third one basically told me to go to the Play Store and download a Task Killer. The last one I called told me to bring the phone to the service center so they could "observe it". That doesn't sound good to me.

Question

My question is can technicians actually tell if the battery has the capacity it claims it has and if the battery is healthy? I want to go prepared to that service center if the observation they'll be doing will be to look at my "Battery" setting only and asking me what I've been doing only for them to say that's what been draining my battery.

Also can 3G drain a battery within 4 hours if a battery has 4000 mAh? 

Note: My phone is a rebranded Innos D9, it's a Cloudfone Thrill 430x.
Multiple things with your issue:


I would never trust a tech who recommends a Task-Killer for this problem. Disqualified 100%.
Same for the one claiming all Android phones last for only one day. That's bogus. It always depends on usage. I have one lasting 7 days (well, no SIM card inserted, not WiFi activated -- it's rather used as a stand-by computing device for some apps -- but it's a phone, and it runs longer than 10h ;), and my "standard phone" ends up at ~75-80% after 14h usage.
tech #2 and #4 sound more reasonable: any app (especially if "consuming push services") can be the cause for such a problem (see below), and monitoring battery usage is a generally good idea (that tech I would trust most from the ones you mentioned)
Different kind of usage causes different battery drain.
Sometimes processes go havoc and drain the battery quite fast. I had that "big surprise" multiple times already, last time yesterday: went down from ~80% to 5% and "emergency shutdown" within ~2h while in my pocket -- I only noticed that by the "beep" on shutdown. But this should not be a regular (daily) issue; I experienced this maybe 4-5 times in a year with my current device.


recommendation: Take a look at the battery statistics (usually found in Settings &rarr; About Phone) to determine which app(s) are causing the most battery drain. Maybe you've got some "bad egg" on your device, if this happens regularly.
second recommendation: Take a look at How to deal with (orphaned) WakeLocks? -- maybe that's what affects you. Again, this could be a mal-functioning app.

battery only lasting for ~10h is nothing rare with heavy usage. Remember these are no longer "dumb-phones", but rather "pocket-computers which also can make phone calls".


Useful resources also include:


How can I tell what is really draining my battery?
How to debug a battery issue?
How can I find out what causes “Android OS” to consume battery?
Follow the tag battery-life