How do you know if your supply is diminishing?

I'm constantly worried about my BF supply.  I go back to work in two weeks, so I've been trying to pump in between BF sessins to build a stash.

Mason nurses for 40-45 minutes and then I will pump about 1 hour after nursing him.  I usually will only get 1.5 ozs when I pump though.

The reason I'm worried about my supply is because my breasts are always soft now and sometimes when Mason nurses he will get mad and pull off the breast.  I will try to express some milk from my breast but hardly anything comes out.

I've been eating lots of eggs, oatmeal, taking fenugreek and blessed thistle and drinking lots of water.  But, I haven't noticed any increase so far.

Do you think my supply is tanking?  What else can I do?  Or does it take awhile for these things to work for my supply to increase?