Identifying the different pests is fairly simple if you know what you're looking for, here are a few tips.
Spider mites- The leaves will be sticky, and the undersides will almost look dusty. If its a really bad infestation there will be visible webbing.
Mealy Bugs- Small white fluffs, looks almost like mini- cotton balls. the leaves will be sticky as well.
Scale- Small little dark brown bumps on the stems and back of leaves, sticky residue on the stems/leaves.
Soft scale- yellowish flat buds along the main vein, on underside the leave.
Aphids- little green insects usually on new growth and flower buds.
White fly- little white winged insects on the back of the leaves. Attracted to many flowering plants.
If your plant still isn't happy and and non of these appear to be on the stems/leaves, I would suggest checking out the roots as well. Root mealy bug can cause yellowing of the leaves and leaf drop, if you take the plant out of its pot and the roots have white fuzz/film all over them you likely have root mealy bug.
So now that I have scared you and you're out inspecting your plants with a magnifying glass, don't fret I have solutions for you.
For Mealy,soft scale,aphids,and spider mites I dilute rubbing alcohol in water and mix in a bit of baby oil. The baby oil will coat the bugs and prevents them from breathing, and the alcohol will dry their little bodies out. I use a ratio of 1/3 alcohol, 1/3 oil, and the rest water. Mix it up in a spray bottle and spray the whole plant, including stems, and undersides of the leaves. For root mealy, allow the soil of your plant to dry out completely then using water and some dish soap fill up a bucket/sink and submerge the plants pot in the mix. If your plant is too big for to do that, simple pour the mix in like you were watering it.
To get the best results repeat this once a week for 4-5 weeks, this way you are able to kill all the insects and any larvae or eggs that may be hatching. Again gross, but necessary to know. Luckily its still summer out and you can do this outdoors ( in a shady area as to not sunburn your houseplants) I don't know how many times John has went to use the shower in the winter and has been greeted by buggy,oily plants. Hopefully this blog post will help save some sick and on the verge of being thrown out plants.