One possibility is with RP-HPLC and one standard. But one affordable method is just buying one ELISA kit like this one (http://www.biooscientific.com/Antibiotics/MaxSignal-Amoxicillin-ELISA-Test-Kit and http://www.bio-equip.cn/enshow1equip.asp?equipid=45921&division=2535) and just follow the instructions. In the case of soil and residual water samples, you can just add 5 volumes of PBS, resuspend very well, incubate of an hour, centrifuge to 12 0000 g for 10 minutes and use the supernatant as your sample taking into account the volume of PBS you are using. I hope this help.