Amoxicillin at a concentration of 20 mg per ml has been reported to be appropriate for skin testing. A range of concentrations has been reported for testing to amino-penicillins, and 20 mg per ml is at the "high end" of that range. Concentrations varying from 2.5 to 25 mg per ml have been used.
If you have difficulty obtaining an intravenous solution of amoxicillin, you could employ ampicillin as an alternative at a concentration of 2.5 mg per ml (the intravenous preparation). The 20 mg per ml concentration could produce "false-positives". Nonetheless, there is caution involved if you use the ampicillin because the structures of ampicillin and amoxicillin are similar, but not identical.
In addition, when performing skin tests to amoxicillin or ampicillin, since concentrations have not been validated over large numbers of patients, if negative one still should perform a graded oral challenge when administering the drug itself.
Drug allergy management in 5 steps (click to enlarge the image).
Brockow, et al. Skin test concentrations for systemically administered drugs - an ENDA/EAACI Drug Allergy Interest Group position paper. Allergy 2013; 68: 702-712.
Amoxicillin skin testing - AAAAI Ask the Expert, 2014 http://bit.ly/1h1Vw5J
Image source: Penicillin nucleus, Wikipedia, GNU Free Documentation License.