Depending on your plant material I would think that one of the quickest ways would be to do an acetocarmine squash preparation of young root tips and then just count chromosomes.  Any good plant microtechnique book will have the method.  Jensen's Botanical histochemistry, for instance.  It takes some practice, but it is quick once you get the method down. 

A fast alternative is flow cytometry. If you use the diploid as a control, the absorbance peak in the haploid tissue will be halved.

Don't know which plants you are working on. Attached paper provides another method for doing so, you can use as a reference:

Appl Spectrosc. 2012 Apr;66(4):447-50 Selection of haploid maize kernels from hybrid kernels for plant breeding using near-infrared spectroscopy and SIMCA analysis. [http://www.ncbi.nlm.nih.gov/pubmed/22449327]

use flow cytometry. you can identify the haploid plant with other (diploid, triploid, tetraploid etc).

How about counting the chloroplast number in stomata guard cells? see:

http://www.tandfonline.com/doi/abs/10.1080/00087114.2012.760880?journalCode=tcar20#preview

Use flow cytometry with control plant.

The easiest method may be counting the chromosomes. If it doesn't lead to the result, as others have said it, flow cytometry or FACS can be an option. A document attached herewith may be helpful.

The fastest screening is flow cytometry with a diploid parent plant as a standard. However, if you do not have a flow cytometer available there are alternatives like chromosome count, stomata measurements etc.

U have to use flow cytometry. If you use the diploid as a control, the absorbance peak in the haploid tissue will be halved. 

Yes, you can. In a population, first identifies dwarfish stunted (or small) and sterile plants and then proceeds to the confirmation of ploidy level through the flow cytometer

You can use cytogenetic analysis. You can count the number of chromosomes in dividing cells of root meristem (in metaphase of mitosis) of plant-regenerants received in anther culture

Counting Chromosomes is the simplest procedure as suggested by Svetlana Turasheva

In the case of rice, if you need to do an fast field selection, you can compare the fenotipics caracteristics. You can take as a guide the size of plants and fertility (0%). Compare the mother vs your possible haploid plants. Comunmete haploid plants are dwarfed and sterile. Then you can verify (check) your observations by flow cytometry and chromosome counting. Sorry my english is not good.

Phenotype of the seedling looks smaller than diplod. We can identify those plants, and collect tissues for cytological study to count chromosomes

 just observe the plant phenotypically you will see the spikelets will be of half the size ( in haploid ) than of actual size of spikelet ( diploid ). this is the best, easiest and simplest method of identifying the haploid rice plant. to cross validate the results you can go for flow cytometry or root tip analysis.