There is no differentiating between mobile and portable ID's with ASP. Most of the 7 digit ID's start with 450**** but there are some of the older radios that start with 100****. It just varies based on the radio age (when the ID was issued). That ID stays with that serial number/radio so if a radio/car is turned in and goes to another troop, the console alias is changed with AWIN to display the correct unit ID in the Communications center console. Almost all ASP console ID's start with 947****
On that note, I do believe counties are issued certain blocks of ID's, for example, almost all AWIN radios in Garland county start with 470****.
Recently, due to the amount of radios on the system, I have noticed they have moved to an 8 digit ID scheme. Most starting with 145***** (that I have noticed).
@dave3825 makes a VERY valid point. You can usually tell by the sound/background noise if its mobile or portable.