Total ink 250% CMYK... visible on screen + how find darkest pixel (PS-CS-6)?

This is an outdated and very uncertain way to do it. The modern way is icc profiles, and what they should have done is give you a CMYK profile to use. Profiles have max ink built in, so that when you convert to it you can't go over. However, subsequent adjustments may put you over again.

Anyway, you can preview ink limit in Acrobat and InDesign. In Photoshop I think the only way is to use the Info panel.

If, to satisfy your curiosity, you would like to locate the darkest image point (not most ink volume*):

At the bottom of the Layers panel, select Adjustment Layer > Threshold. You will be presented with a histogram, and below it, a slider. Move the slider all the way to the left; the image area will go completely white. Inch the slider to the right until the first black patch appears.  Choose the Color Sampler tool to click on the patch. The Color Sampler will mark the site. You no longer need the Threshold layer, so drag it to the Trash. The point you marked will remain. You have now located and marked the darkest spot in the image. 

* For example: 70% cyan is darker than 100% yellow

As a “best practice” I don’t recommend changing total ink limit via selections/edits, however the following link does let you visualise where ink limits are exceeded and how much area is affected:

http://www.curvemeister.com/downloads/cmyk_tac/index.htm

I too agree with the other posts that this is best handled via an ICC profile or perhaps better a Deviclink ICC profile, converting from a source profile to a matching output profile (such as ISO Coated as source and ISO Coated 300% as a destination).

Keep in mind that despite the total ink limit being “baked into” the profile, different rendering intents may create a conversion greater than the ink limit in the profile (as can edits made in CMYK after the conversion).

For standard printing conditions, there are various total ink limit profiles available, however it all depends on the specific CMYK space in question.