|Snape appears to have some product build up.|
With the Curly Girl method, you don't want to put anything in your hair that cannot be removed by a conditioner wash (cowash). The first and most important thing to avoid is silicones (often referred to as cones in the cyber hair care universe). Some silicones are O.K. to use.
How to identify a silicone? Silicones end in -cone, -conol, or -xane. Don't be fooled by ingredients that end in -one, those are usually preservatives. Silicones cannot be removed by cowashing, and should be avoided with the Curly Girl (CG) method.
Partially Water Soluble Silicones- Partially water soluble cones are easier to remove. IMHO, if you use a non sulfate shampoo (low poo) on a regular basis, you don't need to worry so much about partially water soluble silicones. Amodimethicone (A.K.A the A-cone) is the one most commonly seen. I got a lot of the info in this post from this link.
Cyclo-silicones- These evaporate off the hair. In the process of evaporating, they may cause the cuticle to buckle. At one point on the message boards of NaturallyCurly.com, people thought the cyclo-silicones were building up. They weren't actually building up, the change in the way their hair felt was from the buckled cuticle. The Curl Chemist says a low poo should restore the hair. See this Curl Chemist article for more info.
Hexamethyldisiloxane (It is a shame this one doesn't also have a Cyclo in the name)
Water Soluble silicones- These rinse right off of your hair and are allowed in the CG method. If your hair is fine and very prone to being weighed down, you may still want to avoid them. Water soluble silicones will weigh down some fine hair. The first two on the list are the most common.
Anything with a PEG- or PPG- in the name.
Hydrolyzed Wheat Protein Hydroxypropyl Polysiloxane
Lauryl Methicone Copolyol
Silicone Quaternium 17
Mineral Oil (A.K.A. petrolatum)- Some people can use this without any problems. Others, (like me) find it hard to remove. It makes my hair oily and heavy. If you are trying a mineral oil containing product, you may want to try it on a small section/curl first. That way if things go wrong, you don't have a whole greasy head.
Waxes- I'll have to come back when I have a list of ingredients that are waxes. I believe they are mostly self explanatory. Waxes can be so hard to remove that even sulfate shampoos can't remove them. This will vary from person to person. For some people, they are not difficult to remove. If you have wax in your hair that is difficult to remove, try doing a diluted apple cider vinegar rinse. Apple cider vinegar rinses can sometimes remove waxy build up.
Castor Oil, PEG castor oil- Same ruled apply as for mineral oil. Straight castor is popular for hair care to add moisture (mostly with type 4 kinky/curly hair). I've read there is different qualities of castor oil that make a difference. I don't know too much about using the oil straight. I'm more familiar with this as an ingredient. Works for some, hard to remove for others. PEG castor oil is easier to remove. My hair hates castor oil in all forms.
Polyquats- Which polyquats are likely to build up? Depends on which expert you ask. All agree that polyquat-4, common in mousse, is easy to remove. However, there have been several reports on the Wavy Hair Community of build up when using the polyquat-4 containing Aussie Instant Freeze Gel. So, it looks like polyquat-4 may also cause build up in some people. These links have more info on polyquats in hair products. You will need to experiment to find out which polyquats build up on your hair and which are O.K.
Curl Chemist on Polyquats
No-Poo Jillipoo talked to several experts about polyquats.
So, if you avoid everything on this list, does that you will never get build up? Nope. Some random things build up for some people. Jojoba oil and aloe build up for me. Also cationic conditioners are meant to adsorb (cling) to hair. They are usually washed off. But sometimes, some continue to cling to hair and build up. A low poo will remove this build up. This link has more on how conditioners can build up.
What does product build up look like? For me, my hair looks like it has a fake shine. It looks slightly oily. It looks somewhat wet when dry. My hair looks heavy and coated. My hair will have less bounce. I believe for some people product build up can cause frizz.
See this link for a list of sulfates to avoid.