It depends on the kind of sample you have. For example, if you have purified hemoglobin (or the hemoglobin just coming from isolated erythrocytes) you can evaluate it by simple absorbance measurement for example at 413 nm (I use this wavelenght) and then use the extinction coeffient at 413 nm (you can find the different coefficients just googling). If you want a more accurate assay, especially in a complex mixture, you can use the cyanmethemoglobin assay or the azide methemoglobin assay. Unfortunately I have nothing more than name of the assays, since I've just studied them during classes...I hope it might help you searching for something more specific...

There are several options, depending on what your goal is. If you want to just get total Hb, as noted above, in simple OxyHb solutions, you can use the Oxy peak around 413/414 nm. However, if it's likely you have a mix of Oxy and Met (which is generally likely), the isobestic point at 409/410 is better for total Hb.

Alternately, the cyanmethemoglobin assay can be used to assay total Hb:

http://www.sciencedirect.com/science/article/pii/S0305049197002307

If you want to know the make up of your Hb solutions, how much oxy how much met, then using multiple wavelengths is best, such as the Winterbourn method:

http://www.sciencedirect.com/science/article/pii/007668799086118F

We have used the acid haematin method to determine haemoglobin levels - the advantage over the cyannethaemoglobin method is that it does not require the use of cyanide. From memory we just diluted the blood with 0,1M NaOH and read the absorbance at the lamda max and compared this with a haemoglobin standard from haematology treated in the same way. You didn't say what the sample matrix was or what levels you are looking at. Also I am not sure how the search engines treat hemoglobin and haemoglobin?