usage: inform positively and with certainty and confidence; "I tell you that man is a crook!"
3. assure, persuade
usage: assure somebody of the truth of something with the intention of giving the listener confidence; "I assured him that traveling to Cambodia was safe"
4. see, check, insure, see to it, ensure, control, ascertain, assure, verify
usage: be careful or certain to do something; make certain of something; "He verified that the valves were closed"; "See that the curtains are closed"; "control the quality of the product"