First and foremost, has the growth factor been proven to be bioactive? Look for a luciferase assay and EC50 values or cell-based assay data. Is the data available for you to review yourself? Is this data available for every lot?
The higher the purity, the lower the contaminants, and the better for your assay. Look for purity assessment by multiple methods such as SDS-PAGE, endotoxin levels (LAL assay), mass spec, and analytical reverse-phase chromatography.
Lyophilized proteins are usually more stable than proteins in suspension, look for proteins shipped in lyophilized forms, but check appropriate stability and recovery checks have been performed.
Animal-free and animal-derived material free are not standardised terms, when buying animal-free proteins make sure you check the manufactures policy for what animal-free really means.
If you require extra information or help will you be able to quickly get in touch with a growth factor expert? Are any guarantees offered?
If you are performing a large study and want to ensure lot consistency look for suppliers who support stock reservation, bulk ordering and custom vialing.