You can check the effect of TGF-beta by western blot of some of the phosphorylation levels of any important protein molecule (Ras/MAPK, RhoA/ROCK1, PI3-K/Akt, p38, or JNK pathways). I will suggest phospho-Akt (S473 or S308).
OR you can check any target gene of these pathway by Real time PCR.
However, treat the cells after a serum starve course of 16hr. It helps.
As suggested by Goodwin, co-adding a neutralizing antibody is a good control.
If you specifically want to assess if your TGFb is active, a common way to do this is to add it to Mink lung epithelial cells, which are known to be growth-inhibited by TGFb. Here's a good review of the various protocols for assessing active TGFb ligand levels (I've also attached this as a file):