You can definitely check if encryption is enabled for a particular field, even if that field lives within an org that doesn't have Platform Encryption provisioned.
The only trickiness is that when Platform Encryption isn't provisioned onto an org, calling Apex Describe on an sObject won't return the IsEncrypted field as part of its result.
You can work around this by checking for both null and false in your Apex code.
Here's a short example of a Visualforce page that utilizes Apex to generate a list of every field associated with the Account sObject, and it's encryptability. You can do something similar in your code to branch based on whether encryption is available for a particular field.
Id — Encrypted: false
IsDeleted — Encrypted: false
MasterRecordId — Encrypted: false
Name — Encrypted: true
Type — Encrypted: false
ParentId — Encrypted: false
BillingStreet — Encrypted: false
BillingCity — Encrypted: false
BillingState — Encrypted: false
BillingPostalCode — Encrypted: false
BillingCountry — Encrypted: false
BillingLatitude — Encrypted: false
BillingLongitude — Encrypted: false
BillingGeocodeAccuracy — Encrypted: false
BillingAddress — Encrypted: false
ShippingStreet — Encrypted: false
ShippingCity — Encrypted: false
ShippingState — Encrypted: false
ShippingPostalCode — Encrypted: false
ShippingCountry — Encrypted: false
ShippingLatitude — Encrypted: false
ShippingLongitude — Encrypted: false
ShippingGeocodeAccuracy — Encrypted: false
ShippingAddress — Encrypted: false
Phone — Encrypted: false
Fax — Encrypted: false
AccountNumber — Encrypted: false
Website — Encrypted: false
PhotoUrl — Encrypted: false
Sic — Encrypted: false
Industry — Encrypted: false
AnnualRevenue — Encrypted: false
NumberOfEmployees — Encrypted: false
Ownership — Encrypted: false
TickerSymbol — Encrypted: false
Description — Encrypted: false
Rating — Encrypted: false
Site — Encrypted: false
OwnerId — Encrypted: false
CreatedDate — Encrypted: false
CreatedById — Encrypted: false
LastModifiedDate — Encrypted: false
LastModifiedById — Encrypted: false
SystemModstamp — Encrypted: false
LastActivityDate — Encrypted: false
LastViewedDate — Encrypted: false
LastReferencedDate — Encrypted: false
Jigsaw — Encrypted: false
JigsawCompanyId — Encrypted: false
CleanStatus — Encrypted: false
AccountSource — Encrypted: false
DunsNumber — Encrypted: false
Tradestyle — Encrypted: false
NaicsCode — Encrypted: false
NaicsDesc — Encrypted: false
YearStarted — Encrypted: false
SicDesc — Encrypted: false
DandbCompanyId — Encrypted: false
BRHdjkSmZB__CustomerPriority__c — Encrypted: false
BRHdjkSmZB__SLA__c — Encrypted: false
BRHdjkSmZB__Active__c — Encrypted: false
BRHdjkSmZB__NumberofLocations__c — Encrypted: false
BRHdjkSmZB__UpsellOpportunity__c — Encrypted: false
BRHdjkSmZB__SLASerialNumber__c — Encrypted: false
BRHdjkSmZB__SLAExpirationDate__c — Encrypted: false
BRHdjkSmZB__Encrypted_Text__c — Encrypted: true
BRHdjkSmZB__Encrypted_Text_1__c — Encrypted: true
BRHdjkSmZB__Encrypted_Text_2__c — Encrypted: true
BRHdjkSmZB__Encrypted_Text_3__c — Encrypted: true
BRHdjkSmZB__Encrypted_Text_4__c — Encrypted: true
BRHdjkSmZB__Encrypted_Text_5__c — Encrypted: true
BRHdjkSmZB__Encrypted_Text_6__c — Encrypted: true
BRHdjkSmZB__Encrypted_Text_7__c — Encrypted: true
BRHdjkSmZB__Encrypted_Text_8__c — Encrypted: true
BRHdjkSmZB__Encrypted_Text_9__c — Encrypted: true